Doctors have launched a new battle against euthanasia ahead of a crucial vote in parliament, arguing that seriously ill patients who beg to end their lives often go on to change their minds. Peers will vote on Friday on a private member's bill that would allow a terminally ill adult to ask a doctor to help them die, so long as they were judged to be suffering unbearably.

Doctors opposed to what is termed physician-assisted suicide argue it would lead to pressure on vulnerable people not to be a burden to their carers, and they are rallying patients who have suffered serious degenerative illness in support of their argument.

David Williams, 51, a father of three from Cardiff, is backing the campaign. He was a 35-year-old successful service manager for a car dealership when he began suffering severe back pain, and was eventually diagnosed with a tumour on the spine. Williams was told he would be in a wheelchair by 37 and most likely dead by the time he was 40. He had surgery just as his third child was born.

Bedridden, in terrible pain and fearful of the distress he was causing his family, Williams said if doctor-assisted suicide had been available he would have considered it 'purely on the basis of the suffering my wife was going through'.

Williams was eventually admitted to a hospice in nearby Penarth and treated by Baroness (Ilora) Finlay, the renowned expert in end-of-life care. His pain was made bearable and the cancer went into remission. Williams now thinks he was probably depressed by his illness: 'I had a job that a lot of people would die for: I had a nice house and three lovely children, fantastic wife and everything was great. And then it all went wrong. Things do flash by you at that point.'

Williams has another reason to be glad he is alive. A few years after his recovery, his wife contracted liver cancer and died suddenly. Had he opted for suicide, he points out, his children would have been orphaned.

'I used to look at the baby in the cot and think "I can't do this." I look at him now, and I can see me in him: and I just thank God I didn't do it,' he said. 'I am more than grateful that I am here for [the children] now.'

Williams's cancer recently returned and he is in a wheelchair. He is supporting the pressure group Care Not Killing, of which Finlay is a member, and vehemently opposes the bill: 'I do worry if they pass this law that it would be used for the wrong reasons. People would be put under pressure because when you are that ill, you just want to do whatever is the easiest thing.'

The private member's bill, drawn up by Lord Joffe, has its second reading in the Lords on Friday. It is unlikely to pass without government support. However both supporters and opponents believe that if it fails it will be reintroduced to the Commons in a similar form, and possibly with greater momentum.

David Cameron, the Tory leader, has written to opponents pledging to vote against the bill on the grounds that 'terminally ill people may feel pressurised into ending their lives'.

Supporters of the bill argue that in the US state of Oregon physician-assisted suicide has been introduced without problems. Writing in The Observer today, the ethics expert Baroness Warnock argues that while assisted suicide might not be needed if palliative care were perfect, 'this is a distant prospect and I do not believe that everyone would prefer palliative care. There are those for whom it would be a nightmare and who would prefer death to the drawn-out process of being kept alive and conscious'.

Supporters cite the case of Diane Pretty, the motor neurone disease sufferer who went to court seeking the right to die at a time of her choosing rather than in what she feared would be a distressing manner as the disease advanced.

However Peter Saunders, a general surgeon and campaign director of Care Not Killing, says while cases like Pretty's are high profile, Williams's experience may be more common.

'Suicidal thoughts are not uncommon at the time of first diagnosis, and before people have had the opportunity not just to hear about what palliative care is offered but to experience it,' he said. The minority who might want to die, he said, did not have the right to a change in the law that could put others at risk.