In the spring of 2001 my wife caught her right foot on a doorstep and fell headlong, badly damaging her left hand. It recovered, but two months later it began to spasm, fingers shooting out without warning in all directions. The doctor diagnosed a trapped nerve. But then she had three more bad falls, and throughout the next summer she often found herself suddenly staggering into furniture. It was becoming obvious that the problem wasn't simply a trapped nerve, so she was referred to a neurologist, who ordered a scan. I went to see him in December, and though he hadn't yet received the full results, I think he was trying to warn me that the problem might be serious. And it turned out to be so.

Elizabeth was diagnosed with motor neurone disease the next spring. We were immediately inundated with experts - a motor neurone disease nurse, occupational therapists, physiotherapists. They arranged for a wheelchair. They were positive and constructive and seemed to assume that we could cope with all the problems that would come up. But Elizabeth wasn't so sure, and wanted to know the worst, so we turned to the internet. Motor neurone disease is caused by an irreversible degeneration of the motor neurones, the nerves that control movement. The course of the disease is different in every patient, but the end is the same: complete loss of mobility, then death. The terrible thing is that sufferers retain all their mental faculties.

Very definitely, and fairly early, Elizabeth decided that she would not wait for this to happen. She just said to me one day, as we were sitting out in the garden, "I'm not going to go on." Apart from the initial shock, I have to say it didn't surprise me. We had both talked about how we didn't want to die the way some of our friends had - one in particular, who had Alzheimer's and lingered on for years after he had ceased to know what was going on. I guess I could have refused to cooperate, and forced her into a nursing home. But I think that would have been disastrous - for her, anyway.

I wasn't there when she told our children what she was going to do, and gradually I began to realise that they understood her intentions rather differently than I did; that our daughter, particularly, was more aligned with Elizabeth's views than I was. So I wrote her a long letter, arguing that I was in favour, but only when it became necessary; only when Elizabeth's quality of life had declined so much it had become intolerable. It seemed to me that she was jumping the gun, talking about doing it fairly quickly, perhaps not even reaching her birthday, which I thought was crazy.

But the disease was progressing fast, too, in some ways faster than I realised. She had been an active, independent woman who used to climb and ski, but before long she was increasingly in a wheelchair. She was a keen amateur painter, but her hands were weakening fast; in October she attended a life class and did her last good pastel drawing. Face and throat muscles are often involved, and she was finding it increasingly difficult to make herself understood. "Love to see visitors," she wrote in a diary, "but feel like weeping with exhaustion once gone." The doctors promised her bladder wouldn't be affected, but that month she choked on a sweet and "with coughing, wet myself thoroughly. Devastating!" By the following spring she was in a wheelchair all the time.

I became a full-time carer. She couldn't get out of bed herself. She could still stand, but not without me holding her. Her feet were very weak, because muscles waste when they are not being used. In the morning I had to take her into the bathroom and lift her on to a special chair rigged up in the bath, then swing her legs over so that it could lower her into the water. Occasionally we missed, and she would end up perched on the edge of the bath. This would often make us dissolve into laughter, but I'm in my 80s now, and if she had fallen to the floor I would have had to call for help. During the day she would buzz about the house in her battery-powered wheelchair. You can still see the damage on the doorframes.

Eventually I was doing all the cooking. I can't stand going to Tesco, but she liked it. Of course, she couldn't reach the shelves. Sometimes I'd lose her and wander around asking shoppers, "Anyone seen my wife?" She read a lot - almost a book a day. She enjoyed good television, her food, her family, friends visiting. She certainly enjoyed the company of our dog. Physically she was deteriorating rapidly, but in terms of her quality of life, I believed there were many things she could still enjoy.

As far as her decision was concerned, I didn't think of it as happening next week or next month. I knew we were moving towards it, but when it was going to arrive I didn't know. Occasionally, Elizabeth had dreadful sobbing jags - not often, perhaps three or four times. She had one once when I was helping her in the bathroom, and I said, gently, "You can always change your mind, you know."

She used to get immensely upset if I said anything like that because she thought I was trying to persuade her not to do it. But I wasn't. I was just testing that she was sure, and trying to get her to wait until it became ... time. Her great surges of emotion (which I have now) didn't change her mind. They were just a symptom of the terrible situation she found herself in: she didn't want to kill herself. She felt she had to, but she didn't want to.

I found out later that she had been writing to a friend, asking for advice on how to do it, about what drugs she could use, and mentioning that she was thinking of slitting her wrists. This friend had consulted medical people and replied that she wouldn't advise anybody to try some of the drugs Elizabeth had mentioned, and that wrist-cutting usually doesn't work.

So Elizabeth had to turn to me for help, and I turned to the internet, trying to buy prescription painkillers without a prescription. The one foreign firm I did manage to contact simply sent me advertising material; our daughter thought that even if they had sent the drug, it would probably have been stopped at customs. So I didn't pursue it, because I didn't want to attract any attention. I also contacted Dignitas, the euthanasia society based in Zurich, but Elizabeth much preferred the option of being at home when the time came.

Eventually we managed to get some morphine linctus and ampoules of diamorphine. It was reputed to be enough to kill a horse. But we had to make sure Elizabeth didn't throw it all up again, so we had to find an anti-emetic as well. I had a friend with mental health problems who had considered committing suicide himself; he supplied us with the anti-emetic - and advised me to clear my computer of all the emails with any mentions of drugs, all the downloads. Committing suicide is legal here, but assisting it isn't, so I spent about a week attempting to wipe the hard drive. Eventually I had to take it to a friend's son, who is a computer expert. Finally everything was ready. It was just a question of when.

I can't remember what we had for dinner on April 21 2006. It was nothing special. Our daughter was with us for a few days, visiting, and we usually had a glass of wine and some nibbles before dinner, and wine with dinner; we had that. But over dinner the talk turned to our preparations, and gradually it became clear that although I still thought she was going too fast, Elizabeth felt now was the time; she said she was ready, and obviously it was good for me to have our daughter there.

So at 10.30pm Elizabeth drank the necessary quantity of the anti-emetic. We went to the bedroom at about 11 o'clock, and helped her into her best nightgown. Elizabeth was very particular about her appearance, always getting me to brush the dog hairs off her skirts - dark skirts, which showed up the dog hairs. If she'd only worn white skirts they wouldn't have shown.

We wiped all the bottles. I was prepared to go to jail if I had to, though I hoped it wouldn't come to that; our daughter, however, we didn't want implicated in any way. She said goodbye to her mother, and gave her a hug. At that point she was all right. Then she went through to the living room by herself, and sat on the sofa, waiting. That was when she began to go to pieces.

Elizabeth broke the ampoules of diamorphine, dissolved them in the morphine linctus, and poured the potion out, into a plastic beaker. She drank it, and lay down. I lowered the bed, then sat at her side and held her hands. She was calm, and I surprised myself - I was calm too. "I hope this damn stuff is kosher," I said - trying to make a joke of it. Then I just sat there, holding her hands and watching her breathing. I thought it would slow, but it didn't.

She was quiet, but then she kept finding that she wasn't going to sleep when she thought she should be. At some point we cracked another joke - neither of us believed in the afterlife. But there was the question, if there was an afterlife, would she be looking down from above on what was going on, or up from below? Finally she said, "I can get to sleep quicker reading my book." And she got her book, and sat up a bit. It was one of her whodunits - I believe it was a Dick Francis. Just a light thing. She had probably read it before. It was difficult to find a book of that nature she hadn't read before. She was a voracious reader, and my encyclopaedia: if I couldn't remember an author or a name, she could.

Eventually she said, "Don't you sit up any longer - you go to bed." It was about three-quarters of an hour after she'd taken the drug. I said, "Well, I'll never get to sleep," but I did as she asked. I just wished her goodnight, and that was it. What do you say?

I told our daughter that Elizabeth wanted us to go to sleep. She went upstairs, and I went to my room - Elizabeth and I had separate rooms because we couldn't get the wheelchair and two beds into our room - and I really didn't think I'd be able to sleep, but I did. I had to get up at about three to go to the loo. Her light was off, and stupidly I didn't realise what I was looking at, that Elizabeth must have turned it off. I thought, "Well, I won't go in, because I'll have to call a doctor first thing in the morning. And I went to the loo and went back to bed."

At five o'clock - oh, dear God, I still shudder to remember the sound of it - her bell rang. I've never heard anything worse in all my life. I rushed through to her. She was retching and heaving and crying, just in a desperate state. She asked for a bowl because she was going to be sick.

She begged me to cut her wrists. Although I so wanted to help her, I couldn't do that. We would just have to use Dignitas. Our daughter, who hadn't heard the bell, came downstairs shortly afterwards. When she saw what had happened, she was absolutely distraught.

Elizabeth threw up for several hours. It took a few days to get back to some sort of normality. Our daughter was a great help, but eventually she had to return to her own family. It was the last time she would see her mother.

Applying to Dignitas is quite a complicated process, for good reason: they want to make sure you're not doing anything on impulse, so you have to provide copies of birth certificates, and doctors' assessments, and Elizabeth had to write them a detailed letter explaining that she was of sound mind, and exactly why she wanted to end her life. I had initially told them that we wanted to set it all up just in case, that we didn't have a date yet, as the time hadn't come, but after our harrowing DIY attempt, I wrote to them and said that we were ready, and wanted to pursue the issue with some urgency. They suggested a couple of dates, and we chose Tuesday June 6.

I arranged for the dog to be left with friends, as we usually do when we go away. But I thought it would be too rough on Elizabeth to start off with the dog in the car. So I asked them to take her the night before. The dog always jumped into a car if it had a door open. Any car at all. But this time she was sitting beside Elizabeth, and didn't move. I had to pick her up and put her in the car. And suddenly Elizabeth was in one of her sobbing moods, and all she could say was, "My poor little dog!" I felt awful, that I might have done the wrong thing, but I don't think I did. It would have been even more distressing for her to watch the dog being dropped off. In some ways that daft dog was a bit of cement in our relationship.

Elizabeth couldn't make the journey all in one - there was no way for her to go to the loo - so we split the journey into stages. Our son met us at the local airport, and a couple of friends helped us get the special wheelchair on the plane. We had initially thought about going as a family, but our daughter was so upset by the first attempt that she didn't think she could stand it. I was glad our son came. I think I would have found it all too difficult on my own.

We flew to Gatwick, where we stayed overnight in an airport hotel. Elizabeth hadn't wanted to say goodbye to too many friends, but some had to know, and wanted see her, so a couple came to the hotel for dinner. The undertow of what was going to happen was always there, of course, but we had a pleasant evening - lots of chat, and laughter.

We arrived in Zurich on the Sunday, and saw the Dignitas doctor first thing Monday. His job was to assess Elizabeth's physical condition, and to assure himself that this was her own definite wish, that we hadn't pressured her. I kept out of it - except that Elizabeth had trouble talking, so in the end I had to chip in and answer for her. The interview was successful, and the doctor agreed to provide the necessary drug.

Then we went out for a late lunch at a cafe and talked. The sun shone. It was beautiful, like an amazing little holiday. We were staying at a fairly good hotel used by businesspeople, and while on one level it seemed normal, on another it was terribly weird. You would be walking along, pushing a wheelchair, and thinking, "We're here for Elizabeth to die." And all these people were just doing their own thing.

The food at the hotel was good, the wine was good, and in our room there was a bath with a wide tiled section at one end. We both looked at it and had the same thought. "You could sit on that," I said. So I helped her on to it, and pulled the curtain, and she actually had a shower at the end, which absolutely delighted her. Because that was one thing she had craved and couldn't have - a lie-down and a soak in the bath.

The next day we had our appointment at the Dignitas flat. Elizabeth had kept a special skirt and blouse, and I dressed up a bit; I'd brought a special shirt and jacket and trousers, but it was too hot to wear the jacket, so I just wore the shirt, which she made a joke about. We went by taxi, and were met immediately by a man who welcomed us warmly, and helped us feel a bit more at ease.

The flat is tiny, with just one room, with a kitchenette. There's a bed with a thick mattress and cushions. Also a round table, chairs, a carpet, and three nice paintings on the wall, which Elizabeth commented on. There was a little dish of chocolates, which they kept offering us; Elizabeth had to eat them so she could keep down what she had to drink. We did the admin first. Elizabeth signed away power of attorney, and signed a document which exonerated Dignitas of anything that could possibly be held against them if the procedure failed in any way.

We were given some water and though we all had a drink, the real reason was to check that Elizabeth could swallow the necessary volume of liquid. It was explained that she would have to take an anti-emetic half an hour before she took the drug, and repeated, several times, that she could change her mind at any time, up to the point where she took the final glass.

She drank the anti-emetic, and we sat there chatting. Then the officiating member of Dignitas asked our son and me to leave the room for a few minutes. Again this was to make sure that Elizabeth's decision was her own, taken in full control of her senses. He asked if we wanted him to go out of the room. Well, I couldn't have said anything, so he stayed. We just talked. He explained that he would take a video of her drinking the drug as additional proof that she had taken it herself. It would be a 20-second video that nobody would see except himself and the police, and then it would be destroyed. In fact, he conducted the whole procedure with the greatest gentleness and kindness and managed to make Elizabeth feel relaxed and at ease, which amazed me.

Finally he said to her, "Are you ready?" He asked her if she would like to move to the bed, and she said yes. So our son helped her - he was doing the physical job, and, I feel now, was maybe more caring in what he said to his mother at the end than I was. Then he sat with her. I couldn't. They adjusted the bed; she wanted it a bit more upright. He said, "If you drink this, you will die." It was her final chance to change her mind. And she said, "That's all right. I'll take it."

It's very bitter stuff, apparently, but she started to drink it quite successfully. Then we perhaps made a mistake, when he took the video, of giving her a straw. She didn't vomit, but she began to cough. Some had gone down the wrong way. And I thought, "Oh, God," and I went over and held her hand, and said, "Don't for God's sake be sick, Elizabeth." I think she had taken perhaps three-quarters of it, and we were frightened that she wouldn't drink enough to kill herself before she passed out. But eventually she began to black out, and it stopped her coughing. She had drunk about nine-tenths of it. And within perhaps 50 seconds of first drinking it she had gone to sleep. And that was it.

He waited about half an hour after she had drunk it to make sure that she was dead. Then he went to the phone and a policeman promptly arrived, a detective sergeant. He examined all the documents, but I wasn't there. I couldn't stay in the room. And I could never have answered all their stupid questions, but our son stayed. He was very composed. After a while he came out and we went and stood outside on the landing, just staring blankly out of the window.

Then this bevy of people arrived. There must have been half a dozen of them. I know one was a doctor, there to make sure that she was dead, and to establish the manner of her dying, but who the hell the rest of them were, I'm not sure. They may well have been police, because it's regarded as a potential crime scene. The funny thing is, if Elizabeth had known what would happen in the hour after she died, she might well not have gone ahead with it, because I'm sure she was examined very thoroughly. It wasn't the kind of treatment she might have wished. But there we are. It's done now.

Our daughter had suggested we book a return flight to Zurich, in case questions were asked as we were leaving England and the police got involved. When we arrived at Zurich airport with a wheelchair and no passenger, some eyebrows were raised, but everyone must have known what had happened. In all the confusion at security I left a cushion behind, but everything else got through without trouble. Our son went back to his home, and our daughter met me at mine, then phoned people up to tell them that Elizabeth was gone. I couldn't speak.

I don't regret what we did, but I am very angry about the way it had to be done. It seems crazy to me that you must either risk jail or go to Switzerland to do something you should be able to do provided you do it yourself and nobody else is involved. It seems wrong that while suicide is no longer an offence, finding a way to achieve it is. Surely it is a right, like any other human right, to dispose of your life as you see fit?

I observed with interest the halting progress of Lord Joffe's assisted dying bill through the House of Lords. I have to say that none of the arguments made against it persuade me. Some people are worried about the possibility of abuse, but I can't believe that it is beyond the wit of good legislators to provide a framework like the one Dignitas operates within: it must be established that the patient is of sound mind, is making the decision without outside pressure, and is responding to an illness that they believe will certainly lead to a quality of life they will find intolerable.

There is also, of course, the argument of the influence that might be brought to bear on doctors, and I know that the British Medical Association recently voted to oppose the legalisation of euthanasia, overturning its previous policy of neutrality. But I do not see a problem if the decision is allowed to rest with the doctor. One of Elizabeth's asked me if we had had our flu injections. We had, so she suggested we shouldn't next year: if Elizabeth caught pneumonia, she would make sure she didn't suffer. Another doctor apologised to Elizabeth that he couldn't help in the way Elizabeth wished. Of course they can't take risks that would jeopardise their careers. But even if the law changed, they shouldn't have to do it if they didn't want to.

As for religious objections about the sanctity of life, I respect that point of view, but it doesn't mean it has to apply to the rest of us. I know I'm not in a minority: a recent poll, by YouGov for the Dignity in Dying group, found that 76% of people were in favour of assisted dying provided there were safeguards in place.

I have decided to tell this story because I don't want anybody to go through what we did, especially that first attempt at home. I'm also hoping to add a bit of pressure, to help to change the law. Dignitas is not cheap. It probably cost me about £5,000, with all the administration, the flights and hotels and cremation, and while I could afford it, not everyone is able to. More importantly, because the person who wants to die has to be well enough to travel, you end up having to go earlier than you'd like. That was the thing that Elizabeth knew, that I refused to see, because I didn't want to rush - that if we hadn't gone when we did it would have been too late.

· Names and dates have been changed