In 1961, an entrepreneurial and rather prescient housewife from New York realised that you need to know what it's like to be fat before you can tell people how to get thin.

You may not have heard of Jean Nidetch. But you will have heard of the slimming group that she invented over four decades ago. This is the busiest month of the year for WeightWatchers, the internationally renowned diet programme, which encourages people to lose their flabby bits by suggesting a food plan based on a simple scoring system and weighing them in at a meeting every week. With the hopeful gusto that comes from any new diet, thousands will be spending January forsaking cake and crisps and totting up the points in a tub of no-fat raspberry yoghurt.

I was three weeks into my own WeightWatchers' diet, wondering if life was too short for another night in with a skinless chicken breast, when I decided to track down Jean Nidetch (or, more accurately, Dr Jean Nidetch - for the cab driver's daughter from Queens, New York, who dropped out of night school because her family was broke has been awarded an honorary doctorate). To my surprise WeightWatchers couldn't help. Dr Jean Nidetch, the press office curtly informed me, was no longer involved with the corporation. However, three months, many phone calls and, bizarrely, an email from her estate agent later, I am half a stone lighter and on my way to a retirement home in Coral Springs, Florida.

Appropriately, the woman who owes everything to a weight problem - her own when she was 38, and 75 million others ever since - has just finished lunch in the communal dining room.

'Why! Hello-ow! Louise from London!' she beams.

Imagine Barbara Cartland crossed with Joan Rivers. Jean is reassuringly trim, with a billowy blonde perm the colour of cooked pasta, and a megawatt smile. She's wearing a glittery top and a ring set with a gobstopper-sized green stone that she will later gleefully inform me is fake. Dr Nidetch may be a tiny bit uncertain on her feet and in need of a walker these days in order to gad about to her nearest poker game but she's in fairly fabulous nick for 84.

Her apartment walls are covered with black-and-white pictures from the days when she used to be met at airports by cheering crowds bearing banners: 'Jean the Queen!' and 'Be lean with Jean!' There's Jean on the front page of the daily newspaper in Las Vegas where she lived for many years after she retired. Jean holding court at Madison Square Gardens on the 10th anniversary of WeightWatchers. Jean with Fred Astaire whom she dated for a while ('I refused to dance!'). On her desk there is a mountain of unopened envelopes from WeightWatchers franchise holders, the men and women who earn a living by running diet meetings. Somehow one of them put the word out that she was celebrating a birthday and literally hundreds have been in touch.

If the living room is cluttered with memorabilia ('Louise! I have 41 albums. That is all I need. You can always buy clothes, you can always buy furniture. But you cannot replace pictures. To me they are the most memorable thing I own'), the small kitchen is spotless. When she's hungry she joins the queue for single residents in the restaurant and hopes she gets to sit next to someone interesting. For emergencies she keeps a stash of cans of soup. In the fridge there's a tub of ice cream - she treats herself to one spoonful every so often - and a bottle of Baileys Irish Cream.

'Would you like a glass?' she asks. 'I like to have one in the afternoons. Trust me. You can't get drunk on Baileys. It just makes you sleepy!'

She fetches some ice and pours the creamy liquid into a tumbler. I privately wonder how old I was when I last drank a Baileys - 17? 18? - and fleetingly calculate how many WeightWatchers points there are in a mugful of the stuff - 723? 856?

'Ooh, yes please,' I say.

'Good girl,' she replies.

The woman once voted one of the most influential people of the 20th century settles herself behind a desk upon which she still has her old Rolodex. WeightWatchers was more than simply the right idea at the right time. Like that other American guru Dale Carnegie, who wrote How To Win Friends & Influence People, Nidetch had an inherent understanding of what makes people tick (or in this case eat), plus a gift for mesmeric public speaking. At the company's 10th birthday party in Madison Square Gardens (the New York State Department re-named Times Square WeightWatchers Square for the whole week to celebrate) she stood alone onstage and addressed 17,000 fans without the aid of a script. 'For two hours!' she tells me proudly. 'Two hours!'

After the event she was mobbed by well-wishers. One in particular still stands out, 34 years later. A woman approached Jean with a baby. Nidetch recalls: '"Just touch my baby", she said. "Because if it hadn't been for you I could never have got pregnant."' The woman had struggled to conceive until she'd lost 80 pounds on a WeightWatchers diet. She was so grateful, she'd christened her daughter Jean.

In 1961, Jean Nidetch weighed 214 pounds precisely. 'Don't ask me how I remember, but I do! When you're my age certain things come back to you like crazy!' she exclaims. Jean, I am coming to realise, loves telling stories, replete with stage whispers and perfectly timed comedic pauses and dramatic exclamations. She is a natural performer.

She was a 39-year-old, overweight tax clerk, married to Marty, a bus driver, who was also overweight. 'I met him in a diner. I was sitting at a luncheonette. Eating. And he was sat next to me. Eating. We fell in love and we loved to eat. We knew every restaurant in Queens that didn't charge for dessert.' They lived in Little Neck, Queens, with their two sons, David and Richard, in a three-room apartment. When money was tight Jean would go to her aunt's chicken farm in New Jersey and bring back eggs to sell door-to-door to the neighbours.

One day when she was shopping in the supermarket a neighbour said to her: 'Hey Jean! You look great.... When are you due?' She's whispering now. 'I will never forget it. She thought I was almost nine months pregnant. I went home and I looked in a full-length mirror in the closet. And it shocked me. Because I never thought of myself as fat. Actually, I had never even weighed myself.'

There was a diet class that she knew of in Manhattan at the New York City Department of Health. 'It was called,' - big intake of breath - 'the OB-ES-ITY clinic.' Another pause. 'What a horrible word!'

When she arrived everyone was wearing their coats. Many of them were wearing sunglasses. She sat down on the last row. And she listened. 'The speaker was thin, which for openers, I didn't approve of. Then she pulled out a portrait of herself when she was fat. And then I believed her.' (Jean would later stipulate that all WeightWatchers franchise holders must once have been fat and be willing to display life-size pictures of themselves from their XXL days.)

The diet-sheet handout was a revelation. 'Most people who are overweight collect diets. It's like a hobby. You save them. You have drawers filled with them. But the whole idea of this one was that you use your brain and treat your body like you would treat a child. It belongs to you and it needs to be nourished.' Jean was convinced. Every Wednesday she would take two buses and the subway over to Manhattan and attend one of the meetings. 'It occurred to me that what was more important than the weight loss' - and Jean, rest assured, was shedding the pounds faster than a bride-to-be - 'was the companionship. The camaraderie. That meant more.'

Her neighbours began to notice the new svelte Jean, and wanted to know her secret. 'And because I love to share I told them.' However none of them wanted to schlep to Manhattan for the meetings and so it was decided that every Thursday everyone would go to Jean's apartment and she'd tell them what she had learned the day before.

And that was how it began. 'They would call and say, "Can I bring my cousin?", "Can I bring my sister?"' When Jean's chairs ran out, people would bring their own. Marty lost weight; so did Jean's mother, Mae. It got to the point where there was no room left in the living room so somebody suggested that she rent a venue. She found some empty storage space, a loft, above the cinema in Little Neck. The rent was $75 a month. 'I charged everyone the price of a movie ticket. Two dollars.'

She gave up her office job and began to hold meetings, three times a day, after breakfast, lunch and dinner. 'Because you cannot talk to overweight people when they are hungry. If you talk to a fat person about losing weight and all she is thinking about is her next meal then she is not listening.'

There was something about Nidetch, this charismatic everywoman, that was to prove hugely popular. Did she realise this was the beginning of a multimillion-dollar business? 'No! No! No!' she wails. 'Not at all. I didn't realise anything. But I watched people who came, under duress. And they would sit there with their arms folded. And I watched them unfold, and I watched them move closer, just as you are doing now, and it moved me. It still does.' Her eyes fill with tears. 'Wow, I realised, they like to hear me. So I am going to tell them. You don't have to lose weight for me. You don't have to lose weight for your mother. You don't have to lose weight for your husband. You have to do it for you. And if you don't want to, that is OK, too. I never in my life told somebody, "You ought to lose weight".'

You might be reading this and thinking Hurrumph, it's only a diet. You might also be thinking Hurrumph, diets never work anyway - look at all those overweight Americans! And you might have a point. But what Nidetch was instinctively doing for her neighbours, and as WeightWatchers grew, many hundreds of thousands of others, was to offer them a sliver of self-esteem and the knowledge that they needn't feel ashamed. That not everyone is as effortlessly skinny as a twig, but you needn't have thighs the size of oak trees either.

'It mushroomed like you wouldn't believe,' she remembers, taking a sip of Baileys. When she wasn't holding meetings in the cinema she was getting into her Studebaker and driving to see wannabe dieters who lived outside New York. She'd make a point of wearing something chic but plain ('I didn't want to distract people from what I was saying by wearing something fancy') and take along a set of scales. At one such gathering, in Baldwin, Long Island, she met a canny business man called Al Lippert who was to become a key player in the company for years to come. They discussed the idea of franchising out the idea: charging people a small sum to set up their own local diet groups. Lippert suggested she go to see his brother, Harry, who also happened to be a lawyer.

Harry asked her what she wanted to call the new business.

'Let's call it "Lose Pounds!"' she said.

'That's awful,' replied Harry.

'"Watch Your Weight!"'

That's even worse, he replied.

'What about WeightWatchers In-ter-nat-ion-al!' said Jean, drawing out each syllable as she retells the story 44 years later.

Harry said that was ridiculous but he stamped the pages anyway and handed her the legal papers. Marty, Jean's husband, was horrified. 'When is this going to be over? I liked you better when you sold eggs,' she remembers him telling her, in a curmudgeonly fashion. Her mother also pleaded with her to stop. 'Why can't you be a manicurist, like me,' she begged. But Jean Nidetch - 40 years old by this point and weighing a whopping 72 pounds less than she used to - had discovered her vocation and life would never be the same again.

Jean pours me a second Baileys and searches for more albums. She pulls out her cook books, once bestsellers, now long out of print, and says she'd like to put together a new book full of photographs. There are certainly enough of them. She went to the launch of every franchise and would take a photographer to record each trip.

'I never felt nervous. I was at home. I am more comfortable in a crowd than I am at a party.'

By the early Seventies there were WeightWatchers branches all over the United States, Canada and Europe, each one run by lecturers who knew what it was like to be overweight and could sympathise with clients. While Al Lippert oversaw the launch of a low-fat food line, Nidetch focused on publicity and writing columns and recipes.

Jean, how rich were you by this point, I wonder?

'I can't tell you,' she replies. 'It never really dawned on me. I still don't know how much I have in the bank.' Unlikely as this may sound, I believe her. She certainly made a lot of money. She was able to buy her whole apartment building in Queens (she decided not to move out of the area because her sons were still in school) and employ a housekeeper and maids. She bought Marty his bus company (it failed - he preferred being a bus driver, not a businessman). But money was never a motivating factor for her and she doesn't sound as though she was particularly savvy about it. When the company was floated on the stockmarket she was told she couldn't buy shares and she didn't persuade her mother to either. She failed to retain the copyright on her cook books.

What she loved was the attention. 'The adoration of the public is better than anything you can imagine. A billion times better than the money. Sure, money helps to pay the rent. But the adoration! The envelopes!' She sweeps her arm over the fan mail on her desk.

'For a while, you know, I got egotistical about it. Oh, I'm Marilyn Monroe, I thought. I am a star. I remember being filled with ego. And then one day I was getting off a plane, surrounded by crowds of people, and my handbag strap broke. I watched my compact fall, then my mirror, my wallet. And I thought - God just told me who I am. I am not Marilyn Monroe. I am a lady who got thin and now I have to tell the world about it.'

Nevertheless, Marty's plump wife who had loved to eat pudding and had worked the Addressograph at the local tax office was no more. Jean Nidetch, president of WeightWatchers, was flying first class all over the world dressed in fashionable fur swing coats and slingback heels. She was appearing on chat shows and mixing with celebrities like Anthony Quinn and Bob Hope. She arrived at functions with security guards. A picture of her in a slinky white evening dress was used to flog tins of WeightWatchers fish.

'How many people do you know who have been on the side of a fish box!' she shrieks with laughter. Her mother hated the tinned fish, but she used to frame the boxes. 'She would go to the store and say, "That's my daughter!"'

Perhaps unsurprisingly, after 24 years together, her marriage ended. 'I said to Marty: "I know you're unhappy and I don't want you to be unhappy. But I am driven. I have to do this and I know you hate it."'

What did her sons think of their newly famous mother? It must have been strange to discover that their mum was not only thin, or, for that matter, thin and famous, but thin and famous and not at home very often either.

'By this time they were teenagers and to be honest I don't think they thought much of it. Your children certainly don't tell you you are great. When you do something exciting your family is afraid for you. But I was never afraid. It always worked out. I loved what I was doing. I still love it. I think about it. I look at these pictures and relive it.'

After 10 years as president she stood down. I ask if there was a falling-out with colleagues like Al Lippert but she claims not. Simply, the airports had lost their appeal and she didn't enjoy sitting behind a desk. She still worked as a consultant but she'd had enough of the day-to-day slog. 'In the end I wished them good health and said goodbye.'

Perhaps she really had had enough - she was in her fifties, with money in the bank. In 1978 the company was sold to Heinz and it's said that Nidetch received over seven million dollars. She sold the apartment block in Queens for six times what she paid for it and moved to the west coast.

There was an audacious romance with an Italian bass player whom she met on a cruise ship and married within a week - 'I called him the Italian stallion but it wasn't to last. We never fought, but we never talked either.' She won't tell me his name because she has never actually divorced him. 'To my knowledge he is still alive. But since I never remarried and neither do I plan to...'

For 22 years she lived in Las Vegas. She played a lot of poker and gave money to good causes. Every year 20 students from disadvantaged backgrounds have their college fees paid by the Jean Nidetch Foundation. Following a substantial donation there is a building named after her at the University of Nevada.

And then after all that good fortune, there came tragedy. In October 2006, her youngest son, Richard, an actor who lived on the top floor of her house, collapsed suddenly from a ruptured stomach tumour. She called 911 and the ambulance took him away. The following day the hospital called. 'He had, they said, "expired".' She whispers the word. Suddenly, the woman who a moment ago was gaily recalling her ill-advised marriage to an Italian toyboy sounds frail and dreadfully weary.

'He lived with me. We talked every night. I will never get over it. My first thought was to damage myself and I thought about it. But although I am not religious I do believe that if you commit suicide you won't go to heaven. I remember reading that somewhere. But when I get there I have to ask God - how come a parent should lose a child? It's like losing a limb, only worse.'

Some days are better than others. 'I don't feel old. I know I am 84. I know this is the finale of my life.' Her eldest son David helped her to move to Coral Springs, which is near his home. Her friends know that she'll answer the telephone between 11am and 11pm, no earlier and no later. Every Saturday she has her hair done at the beauty parlour. Every Thursday afternoon there is a nickel-and-dime poker game. There are her photographs and her grandchildren and a tumbler of Baileys in the afternoons to look forward to.

WeightWatchers is now owned by a European investment firm, which bought the business from Heinz for $735 million in 1999. Every so often she wonders if she's been forgotten and will cold-call the company she invented in a three-room apartment in Little Neck, Queens, over 40 years ago.

The last time Jean rang the head office in New York she asked the unsuspecting woman on the switchboard, 'Who invented WeightWatchers?'

'Oh my God! You don't know?' said the woman. 'It was Jean Nidetch!'

'And is Jean Nidetch still alive?' asked the real Jean Nidetch, a tad mischievously.

'Oh my God! I hope so!' replied the woman. 'Don't tell me she died!'

A word of advice. Should you happen to work for WeightWatchers, and an elderly lady with a ballsy New York accent rings up one day, take it from me. The indomitable and rather marvellous Jean the Queen is still with us, thank goodness.