Gill's stance marks a major shift by the Church of England and was welcomed by groups campaigning for a change in the law to allow for people to be helped to die under strictly limited circumstances.

'There is a very strong compassionate case for voluntary euthanasia,' Gill told The Observer . 'In certain cases, such as that which involved Diane Pretty [the woman who was terminally ill with motor neurone disease and who campaigned for the right to be helped to die], there is an overwhelming case for it.'

His claims were last night seized on by pro-euthanasia groups as evidence that the archbishop is prepared to engage in a debate on an issue that has long divided the clergy.

They described Williams's decision to send Gill to give evidence to the committee hearing Lord Joffe's private member's bill on assisted dying for the terminally ill as 'highly significant' and suggested that it represented a softening of the Church's attitude to mercy killings.

'The archbishop's choice of Gill represents a willingness to enter into a more constructive dialogue than before about this important issue. We hope it will encourage other members of the clergy to speak out openly in support,' said Deborah Annetts, chief executive of the Voluntary Euthanasia Society.

Gill's comments come after Brian Blackburn, a retired policeman who killed his terminally ill wife in a suicide pact, walked free from the Old Bailey last Friday with a nine-month suspended sentence.

The court heard that Blackburn's wife, Margaret, 62, had only weeks to live and had asked her husband to cut her wrists. Judge Richard Hawkins described the action 'as the last loving thing' Blackburn could do for his wife.

Annetts said the case confirmed that Gill was right to call for those involved in similar mercy killings not to be prosecuted.

'Christianity is about compassion, and one has only to look at the sad circumstances in the Blackburn case to recognise that the current law is not a compassionate response,' she said.

'Using the criminal law to determine end-of-life decisions is not medically, legally or ethically appropriate, nor is it a proper public policy approach.'

At the 1998 Lambeth Conference, Anglican bishops agreed that withholding excessive medical treatment when there is no 'reasonable prospect of recovery' was consistent with Christian princi ples. But Gill's new claims, which will be seen as having been endorsed by the archbishop, go much further and will rekindle the debate over the morality of mercy killings.

Gill acknowledged that the Church's views on euthanasia are divided and out of step with the majority of its congregation. 'Anglicans are not united on whether we should legalise euthanasia,' he said. 'The bishops have consistently shown they don't believe in changing the law, but the majority of churchgoers think it should be amended.'

Last week several leading clerics, including the Reverend Professor Paul Badham, who is also an expert on the morality of euthanasia, wrote to the Daily Telegraph arguing that helping the terminally ill end their lives was 'compatible with faith'.

'It worries us when organised religion is not sharing the heartfelt view of mainstream faith-based opinion in this country,' the clerics noted.

Gill stressed he did not believe relaxing the ban on euthanasia was the way forward. 'I feel that would make vulnerable people more vulnerable,' he said, but agreed the issue was a grey area.

END