A internet survey of doctors has found 45% of those questioned believe some colleagues are helping terminally ill patients to die, and risking imprisonment by doing so.
Some countries allow assisted dying
The research, for the Voluntary Euthanasia Society, found 56% of 1,000 doctors surveyed were in favour of regulated physician-assisted suicide.
Just 21% supported its continued criminalisation.
However, the British Medical Association argues most doctors are opposed to a law change.
The survey, which was completed by doctors who visited a website run by the company Medix UK, also found 27% of those who chose to respond to the survey have been asked by a patient for help to die.
The results are published on the day the BMA gives evidence to a House of Lords select committee considering Lord Joffe's Assisted Dying for the Terminally Ill Bill.
The Royal College of Physicians and the Royal College of GPs have both dropped their opposition to the Bill.
Last month Dr Hazel Biggs, director of medical law at the University of Kent, estimated there could be as many as 18,000 non-voluntary medically assisted deaths in the UK every year.
Deborah Annetts, VES chief executive, said: "Doctors are faced with the awful choice of either respecting the wishes of their terminally ill patients and helping them to die in the full knowledge that they could be prosecuted and face imprisonment, or abandoning their patient to yet more suffering at the end of life.
"It is time that we were open and honest about the need for the option of medically assisted dying at the end of life within proper safeguards.
"Only in this way will patients and doctors no longer be victims of this outdated and inhumane law."
Former Liberal Democrat health spokesman Dr Evan Harris also called on the BMA to review its position.
He said: "I am not surprised that so many doctors want assisted dying to be properly regulated.
"When more than a quarter of doctors are being asked for help to die, and around half of doctors are willing to act on these requests, it demonstrates that the current law is both cruel and in part ineffective."
Dr Michael Wilks, chairman of the BMA's Ethics Committee, said: "As the body representing doctors in the UK, the BMA is opposed to euthanasia and physician-assisted suicide because this is the view of the majority of its members.
"Physician-assisted suicide has been debated many times at the BMA's annual meetings and on every occasion the membership has decided against calling for a change in the law."
HELPING PATIENTS DIE
Physician-assisted suicide: A person wishing to die takes their own life with the indirect help of a doctor, for instance by providing prescription drugs
Voluntary euthanasia: A person wishing to die is directly helped to do so by a doctor
Dr Wilks added that the VES survey also demonstrated that doctors were opposed to euthanasia.
When respondents were asked under what circumstances they thought the law should permit voluntary euthanasia, 53% answered 'under no circumstances'.
The ProLife Alliance questioned the validity of a survey based on a self-selecting sample of doctors.
Spokeswoman Julia Millington said "How is it that on the one hand 70% of doctors have never been asked by a patient to assist in their suicide or voluntary euthanasia and yet 45% believe their colleagues may be helping patients to die?
"However, if there is any truth in these alarming claims that doctors are killing patients why aren't we calling for an immediate government investigation?
"If doctors are killing their patients now when it is illegal, how can the pro-euthanasia lobby be confident that doctors will comply with safeguards were the law to be changed?"
Under a legal principle known as "double effect", a doctor can increase a patient's level of pain relief, knowing that it may shorten the patient's life, providing their main intention is to relieve pain.
However, if the intention is to hasten death they can be prosecuted for murder.
The Assisted Dying for the Terminally Ill Bill proposes to give a terminally ill patient the right to request medical assistance to die, providing they are mentally competent, adult, and suffering unbearably.
Dr Iona Heath, chair of ethics committee at the RCGP, said while they were neutral on the wording of the Bill, that did not mean they supported it and the change in the law it was calling for.
She said: "Our role as trusted medical professionals is to advise on the medical consequences of such a change in the law, not to support or oppose it either