Oxford West and Abingdon MP Dr Evan Harris has called for a new Parliamentary ruling on the legality of assisted suicide.

It comes after the deaths of Peter and Penny Duff, from Bath, who suffered from terminal cancer and took their own lives at a suicide clinic in Switzerland.

It was the first assisted suicide since the Lord Chief Justice indicated anyone helping a terminally ill person to organise an assisted suicide abroad would not face prosecution.

Although suicide is no longer a crime in England and Wales, aiding and abetting suicide is a criminal offence punishable by up to 14 years in prison.

A number of legal bids to change and clarify the UK law for assisted suicide in particular have so far been unsuccessful, leading to demands on the Government to issue guidance.

Liberal Democrat MP Dr Harris said: “The Director of Public Prosecutions has never found a basis for prosecution in over 100 cases of assisted dying — including one where the patient was not even terminally ill — so it is time for Parliament to remove the doubt which makes an upsetting situation even more traumatic."

The calls for a change have been marked by high-profile legal bids and a steady stream of publicity about UK citizens who have travelled to Dignitas, the Swiss clinic, to die.

Multiple sclerosis sufferer Debby Purdy nearly forced the issue last month after losing a Court of Appeal bid to clarify the law.

The patient from Undercliffe, Bradford, in West Yorkshire, believes the time may come when the extent of her suffering will make her life intolerable.

She wants to be sure that her husband, Cuban violinist Omar Puente, will not be prosecuted if he helps her travel abroad to die in a country where assisted suicide is legal.

But Lord Chief Justice Lord Judge and two other appeal judges, Lord Justice Lloyd and Lord Justice Ward, ruled Ms Purdy was not legally entitled to the kind of specific guidance from the Director of Public Prosecutions she was seeking.

Ms Purdy's legal bid followed the rejection in the House of Lords of an Assisted Dying Bill in 2006, brought by retired human rights lawyer Joel Joffe.

It also came in the wake of the death of paralysed rugby player Daniel James, 23, who committed suicide at a clinic run by Dignitas in September last year.

The Crown Prosecution Service (CPS) considered bringing charges under the Suicide Act against his parents Mark and Julie James, from Sinton Green, Worcester, who made payments to Dignitas, sent documents and made travel arrangements to take their son to Switzerland.

But the DPP said that although there was enough evidence to pursue charges against the couple with a reasonable chance of securing a conviction, such a prosecution would not be in the public interest.

A poll after his death suggested more than two thirds of people would back a change in the law on assisted suicide.

The survey, carried out by YouGov, also showed 61% would consider ending their life if they were suffering from a terminal disease.

It came after Sky TV broadcast the death of Craig Ewert, who travelled to the Dignitas clinic in Switzerland to end his life.

The assisted suicide issue was brought to widespread attention in 2002 when Diane Pretty, 43, a terminally ill woman, launched a legal bid for her husband to be allowed to help her take her life without fear of prosecution.

She died at a hospice in May that year near her home in Luton, Bedfordshire, shortly after losing her case in the European Court of Human Rights.

Her lawyers had argued that under the European Convention on Human Rights, which guarantees the right to respect for private life and bans inhuman and degrading treatment, she should be allowed to die with dignity, rather than face the distressing final stages of her disease.