A wealthy British couple who both had terminal cancer have committed suicide together at the Dignitas clinic in Switzerland.

Peter and Penny Duff, from Bath, are the first Britons to die at the controversial clinic since the Lord Chief Justice signalled that anyone helping a terminally ill person to organise an assisted suicide abroad would not be prosecuted.

Their death comes as doctors are to be told by the medical regulator to give terminally ill patients a greater say over how and when they die.

In a fundamental shift, draft guidance from the General Medical Council (GMC), seen by The Times, warns that doctors could be struck off if they refuse to withhold potentially life-prolonging treatment.

It advises doctors to give consideration to patients’ wishes if they wish for life support to be withheld or do not want to be resuscitated in the event of organ failure. Where a patient’s wishes are explicit, acting against them “should be deemed to be causing harm”. It adds: “Serious or persistent failure to follow this guidance will put your registration at risk.”

Mr Duff, 80, a retired wine consultant, and his 70-year-old wife are believed to have been helped to end their lives last Friday with an overdose of barbiturates. Mr Duff founded, with Lord Montagu of Beaulieu, the Wine Guild of Great Britain and was executive president of the International Wine and Spirit Competition. He also founded the pressure group Alcohol in Moderation (AIM).

The couple’s daughter Helena Conibear, said that her parents had done a “beautiful and remarkable thing”. She said: “It is an amazing story, but because of the legal issues involved we are unable to discuss it at this stage.” She declined to say whether any members of the family accompanied her parents to Zurich.

Edward Leigh, a former Tory minister, said the law was allowing “euthanasia by the back door”. He said: “On one hand it is against the law to assist a suicide, on the other the Appeal Court says that if you go abroad to do just that you won’t be prosecuted.”

Medical authorities in Switzerland have yet to issue death certificates for the couple and the family have not yet notified the authorities of their deaths, according to the Bath coroner.

Under Swiss law anyone who assists a suicide faces questioning to ensure no crime has been committed. In England and Wales, assisting a suicide remains a crime punishable by up to 14 years in jail, although to date no one who has accompanied a relative to Dignitas has been prosecuted.

The only other British couple known to have committed suicide together at the clinic were Bob and Jenny Stokes, from Bedfordshire, who were both in their 50s when they were helped to die in 2003. Neither was terminally ill. Mr Stokes had epilepsy and his wife multiple sclerosis. Swiss law does not require people seeking assisted suicides to be dying already.

The crossbench peer Lord Joffe said last night that he intended to reintroduce his Assisted Dying for the Terminally Ill Bill. which would let patients take steps to end their lives.

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