The death of a millionaire hotelier and motor neurone disease sufferer will be screened on television tonight.
Campaigners have accused the BBC of helping to promote assisted suicide in the documentary by Sir Terry Pratchett.
But the BBC has denied the screening will lead to copycat suicides, saying it will give viewers the chance to make their own minds up on the issue.
In Choosing to Die, on BBC Two, Peter Smedley (71) is seen taking a lethal dose of barbiturates at the Dignitas clinic in Switzerland.
The film follows Mr Smedley from his mansion in Guernsey to the Dignitas clinic, which over the last 12 years has helped 1,100 people to die.
Following an advance screening of the documentary, a Dignity in Dying spokeswoman said it was “deeply moving and at times difficult to watch”.
“It clearly didn’t seek to hide the realities of assisted dying,” she said. "In setting out one person’s views on assisted dying, it challenges all of us to think about this important issue head on and ask what choices we might want for ourselves and our loved ones at the end of life.”
She added that it also “raised a number of concerns" about the current law. “Censoring the debate will do nothing to help those suffering unbearably and against their wishes at the end of life,” she said.
“Not only are people travelling abroad to die, but there are also those who are ending their lives at home, behind closed doors, or with the help of doctors and loved ones who are helping illegally.
“Much better would be an assisted dying law with upfront safeguards, which would investigate a request to die when the person is still alive and alternative options can be set out.”
But anti-euthanasia campaigners complained about its portrayal of assisted suicide.
Alistair Thompson, a spokesman for the Care Not Killing Alliance pressure group, said: “This is pro-assisted suicide propaganda loosely dressed up as a documentary.” Mr Thompson accused the BBC of repeatedly giving voice to pro-euthanasia views in both fiction and non-fiction programming but failing to offer the opposite view.
The campaigners claim that this is the fifth programme produced by the BBC in three years presented by a pro-euthanasia campaigner or sympathiser.
Other examples include a BBC Panorama documentary fronted by pro-euthanasia MSP Margo Macdonald and last year's Richard Dimbleby Lecture, in which Sir Terry called for the introduction of euthanasia tribunals.
According to Care Not Killing, such portrayals of euthanasia risk creating a “suicide contagion” among the vulnerable.
He said: “The evidence is that the more you portray this, the more suicides you will have.
“The BBC is funded in a different way to other media and has a responsibility to give a balanced programme.”
Choosing to Die was made by author, Alzheimer sufferer and euthanasia supporter Sir Terry.
Before leaving home Mr Smedley tells Sir Terry: “My condition has deteriorated to the point where I need to go fairly shortly.” He is shown in Switzerland taking a lethal dose of the barbiturate Nembutal helped down with a praline chocolate.
Later he is seen choking and asking for water while his wife of 40 years, Christine (60), holds his hand.
As he dies a staff member tells the camera: “He is losing consciousness, very soon the breathing will stop and then the heart.” Sir Terry added: “This has been a happy event.
“He died peacefully, more or less in the arms of his wife, quietly.” But at the end of the documentary Sir Terry said he cannot decide whether he will be able to end his own life.
“I am not sure what I would do if I was there,” he said.
The BBC denied it had any bias in the public debate over the issue. A spokeswoman said the documentary was “about one person’s experience, Terry’s journey exploring the issues and the experience he is going through”.
“It is giving people the chance to make their own minds up on the issue.” She also accused Care Not Killing of “cherry picking” five programmes from among the BBC’s large volume of output. “Five programmes in three years is not a lot of programmes,” she said.
As evidence that the BBC gives a voice to different views on assisted suicide, the spokeswoman said a Newsnight debate will be shown immediately after the documentary. The debate will include people in favour and against euthanasia.
PA