By Adam Brimelow
BBC News health correspondent
Many patients are put under sedation
The recent ruling by the law lords in the case of Debbie Purdy has re-ignited the debate over assisted suicide.
Polls suggest that while a majority of the public would support a change in the law to allow assisted dying, most doctors are against it.
But there is evidence that some clinicians may already be using continuous deep sedation (CDS), as a form of "slow euthanasia".
Research suggests use of CDS in Britain is particularly high - accounting for about one in six of all deaths.
Every year more than 1,000 people are admitted onto the wards at St Christopher's Hospice in Sydenham, south London.
It is at the forefront of research and education in end-of-life palliative care.
Dr Nigel Sykes, medical director, said only a handful of patients each year require sedation to make them unconscious at the end of their lives.
"Deep sedation, in the sense that you are wanting to make someone unaware of their surroundings, they are asleep, comatose, that is something that is required very uncommonly indeed."
Dr Sykes said CDS can be appropriate for patients who become confused and deeply agitated - but only when nothing else can relieve their distress.
But research by Clive Seale, professor of medical sociology at Bart's and the London School of Medicine and Dentistry, suggests the use of CDS across the UK is far from "uncommon".
"The only other two countries where the prevalence has been measured is in the Netherlands and Belgium," said Professor Seale.
"The surprising thing was that in the UK the prevalence of continuous deep sedation until death was very high indeed, 16.5% of all UK deaths."
That is twice as high as in Belgium and the Netherlands.
But while rates of CDS in the Netherlands appear to be rising, the use of euthanasia has declined.
Dr Judith Rietjens, from Erasmus University Medical Centre in Rotterdam, said this shift is particularly marked among GPs looking after cancer patients.
"It seems that there's substitution from the practice of euthanasia to the practice of continuous deep sedation," she said.
The Debbie Purdy case has put assisted suicide back in the headlines
"We can see in our study that those sub-groups where we saw an increase of continuous deep sedation - just in those sub-groups - we saw a lowering of the frequency of euthanasia."
Professor Seale thinks something similar may be happening in the UK.
"There is good evidence from the Netherlands and Belgium to show that quite a lot of doctors who find providing euthanasia very emotionally distressing and ethically difficult, find that providing continuous deep sedation is an easier thing to do," he said.
"In those countries euthanasia is an option - it's legal. In the UK it isn't.
"Whether doctors in the UK are thinking in this way, and nurses as well, is something which is worth exploring more."
There are fears that CDS is being used inappropriately.
Dr Philip Harrison, a GP now based in New Zealand, set out his concerns recently in the British Medical Journal, following the death of his father in Doncaster Royal Infirmary.
He was put under continuous deep sedation without being consulted, and so had no chance to say goodbye to his family.
Dr Harrison reached the hospital two hours before his father died.
"I'm 100% certain he would have been horrified to know that he would never see us even though we were coming," he said.
"There was no reason on earth why he would have wished to have been put to sleep, unless he was obviously distressed or agitated or in pain.
"But there was no evidence he was in pain at any stage during his admission."
Dr Harrison, who has long experience in palliative care, decided not to sue the trust - but he did try to get reassurance that it couldn't happen again.
Despite an apology he is still not satisfied.
"I don't know what the legal term is but to me it was as near to a form of murder that I had come across," he said.
"I have never seen that in my medical practice before. I've seen euthanasia once, but I've never seen anybody being put to death without consent."
Dr Harrison said he is concerned about what could be going on across the NHS in the name of caring and terminal sedation. The truth is, no one knows.
No clear definition
Dr Nigel Sykes said one problem was the lack of a clear definition of CDS.
He also points out that many patients close to death lose consciousness regardless of their medication.
He emphasised the importance of discussing treatment - with the patient whenever possible, with the family, and with specialist colleagues.
"There is really no excuse for a doctor to take the line of consciously using deep sedation as an alternative to euthanasia because he can't think of anything else to do.
"There are sources of advice available because specialist palliative care is now available and accessible across the country."
Dr Sykes said there was a need for further research to establish how much sedation is being used, who is using it, and why.