Doctors with religious beliefs are less likely to take decisions which could hasten the death of those who are terminally ill, a study suggests.
The survey of nearly 4,000 doctors found those with a strong faith were also less likely to discuss end-of-life treatment options with their patient.
The London University research urges greater acknowledgement of how beliefs influence care.
Doctors and campaigners described the findings as "concerning".
Dr Clive Seale of Barts and the London School of Medicine analysed 3,733 responses from doctors practising in a wide variety of fields, with a particular focus on those who worked in palliative care and with the elderly.
The General Medical Council, which regulates the profession, says that while doctors should start from the assumption that life should be prolonged, this should not be at any cost.
While it is illegal to give drugs with the deliberate intention of ending someone's life, doctors may administer morphine or other medication to relieve pain or distress which may have the effect of shortening life - so-called deep sedation.
The GMC also advises that discussions surrounding end-of-life care, including issues such as tube-feeding, hydration and resuscitation, which can all cause avoidable distress, be started with those who are terminally ill well in advance of their dying days.
Those who described themselves as very or extremely non-religious were about 40% more likely to sedate than religious doctors, the study in the Journal of Medical Ethics reported.
Being very or extremely religious was also associated with significantly fewer discussions of decisions with patients around treatment at the end of life.
More than 12% of the doctors surveyed described themselves as very or extremely religious, compared with just over 6% of the UK general population - as documented in the last national survey of attitudes.
However one in five described themselves as very or extremely non-religious, slightly higher than the general population.
The UK medical population differs in ethnic profile from the population as a whole, with a higher proportion of doctors from South Asian backgrounds.
The survey showed that specialists in the care of the elderly were more likely to be Hindu or Muslim than other doctors, while palliative care doctors were slightly more likely to be white and Christian.
The British Medical Association said: "Decisions about end-of-life care need to be taken on the basis of an assessment of the individual patient's circumstances - incorporating discussions with the patient and close family members where possible and appropriate.
"The religious beliefs of doctors should not be allowed to influence objective, patient-centred decision-making. End-of-life decisions must always be made in the best interests of patients."
Professor Mayur Lakhani, chair of the National Council for Palliative Care, said he was concerned by the findings of the paper.
"Core training in palliative care should be mandatory for all doctors to ensure consistency of decision-making, based on best practice and current evidence.
"Decisions must be taken in partnership with people approaching the end of life and those close to them.
Dr Ann McPherson of the charity Dignity in Dying added that while there was growing awareness of the need to put patients' wishes first, the research indicated there was "still some way to go".
"The fact that some doctors are not discussing possible options at the end of life with their patients on account of their religious beliefs is deeply troubling.
"Whilst entitled to their beliefs, doctors should not let them come in the way of providing patient-centred care at the end of life."
David Praill, chief executive of Help the Hospices said the findings highlighted "the maze of complex moral and legal dilemmas that doctors can face when caring for patients at the end of life".
"Doctors need support to ensure that they have the information, skills and confidence necessary to make the best end of life care decisions with their patients."