Last night, courtesy of the BBC, we could watch a man being killed – voluntarily. The much-heralded climax of the documentary Choosing to Die was of 71-year-old Peter Smedley being administered a lethal dose of Nembutal helped down with a praline chocolate (this was in Switzerland, after all). In his comments to accompany Smedley's death, the presenter, Sir Terry Pratchett, declared: "This has been a happy event."

It certainly seems to have been one for Sir Terry, who since his diagnosis with a form of Alzheimer's disease has become Britain's most prominent campaigner for "assisted suicide". It was only last year that the BBC gave Pratchett the great platform of the Richard Dimbleby lecture; the novelist used it to mount a forceful argument in favour of what he calls "the right to die" for those unable to commit suicide without assistance.

It is clear that the intended beneficiaries of Pratchett's campaign must be those who are physically incapacitated – as he fears he will be: the fully able-bodied need not call upon others to kill us. For this reason, any legislators thinking of backing Pratchett's cause would do well to consult the members of the charity Scope. Scope represents the hundreds of thousands of Britons with cerebral palsy, far and away the most common form of congenital physical disability and which affects, to a greater or lesser extent, one in every 400 children born in the UK.

The chief executive of Scope, Richard Hawkes, having observed the head of steam building up behind the campaign for voluntary euthanasia, has sensibly been spending much of his time consulting his members, and other disability groups, about their feelings on this matter. Last month Scope released one of the results, a poll by ComRes, which revealed that no fewer than 70 per cent of disabled people are concerned that the changes in the law advocated by Pratchett would create pressure on vulnerable patients to "end their lives prematurely". Scope has also been conducting focus groups of people with cerebral palsy, in order to add qualitative research to the quantitative findings of pollsters; the charity invited me to attend one of these encounters, and to observe many hours of such discussions from behind one-way glass alongside a group of its full-time employees.

It was an experience both moving and gruelling – though obviously much less gruelling than it must have been for some of the discussants, for whom intelligible speech came only with the greatest effort. One of the participants, a 45-year-old man, was completely fluent –he was the least affected by CP. He didn't want me to quote him by his real name, partly because he does some work for the NHS, so I will call him Bill. "Bill" told the meeting: "Five or so years ago I was very pro the legalisation of assisted suicide, but I've changed my mind – even though I do know what it's like to be in pain every day. I can see in 15 years or so my relatives saying, 'Bill's depressed, he can't work, but hey, assisted suicide is an option.' I struggle to see in practice a system of assisted suicide that will guarantee vulnerable people don't end up being assisted quite aggressively. In other words, I'm not sure that so-called informed consent will work in the way advocates of assisted suicide claim."

It may seem rather bleak to suspect one's own relatives of exerting insidious influence of that sort – albeit dressed up as the most altruistic concern; but unfortunately the country does not consist entirely of families happy to pay whatever it takes to keep an elderly relative going even if that means that their hoped-for inheritance is swallowed up by that same relative's care-home fees. It is not necessary to speculate as to whether the old lady in the care home might feel obliged to "do the decent thing" for her descendants; one need only read a letter to The Times from a 90-year-old named Margaret White in which she wrote: "I am happy here in the nursing home with no wish to die, but were voluntary euthanasia to be made legal I would feel it my absolute duty to ask for it as I now have 19 descendants who need my legacy. I am sure I am not alone in this resolution."

The issue is not just one of family pressure. The Government, too, is under enormous financial constraints as a result of the rapidly growing costs of care for the old and physically dependent. I would not claim that it is financial consideration which persuades some hospital doctors to place "do not resuscitate" signs at the foot of the beds of the disabled, without consulting them: but still, it happens, and it speaks of a cast of mind that already seriously underestimates the value of the lives of those who have never been able-bodied, one which would seize on the idea of assisted suicide as a happy outcome for all concerned.

Thus Andy, another at the cerebral palsy meeting, told how when he was recently hospitalised, a doctor walked up to his bed and said, "I think we should seriously consider your future." Andy argued that "we are already pressurised not to demand proper medical care, and the pressure would become even greater if assisted suicide were legalised. It could be a case of 'You should have yourself killed, mate. Then we won't have to pay your disability living allowance.'"

The opinions expressed by "Bill" and Andy had a striking effect on Valerie Lang, a 71-year-old from Islington in north London. Valerie admitted that her mind had been "substantially changed" by what she had heard. At the end of the meeting she said: "I know there will come a time when my body won't have given out, but my patience jolly well will; and I will want to go to Dignitas when that happens. But I have been reminded of how vulnerable people, less strong-minded than me, can be open to suggestion."

Valerie had told the group earlier that she had spent 20 years "falling in and out of depression" until therapy had enabled her to come to terms with her cerebral palsy. Since then she had had a remarkable life, despite her severe disability: she had become a senior research officer in the Civil Aviation Authority's Economics Division and since her retirement had, as she put it, "sat on many committees, but have now reduced these to about six". It is easy to see how under a system of legalised assisted suicide, Valerie's earlier years of depression at her condition might well have been ended; and as Richard Hawkes points out, "the thing about suicide is that it doesn't allow for second thoughts".

Something else Valerie said at the meeting struck me as dreadfully sad: "When I was younger, if I fell over in the street two or three people would always rush to pick me up. Now people just walk past. I am no longer even part of the scenery."

That is the state of mind, on both sides, in which assisted suicide for the severely physically disabled could so insidiously turn from being a liberating option into something more like an oppressive social obligation. I have no doubt that Terry Pratchett's campaign has good intentions; but for the very people he most means to help, they could pave the road to Hell.

d.lawson@independent.co.uk