The UK will cross a “legal and ethical Rubicon” if Parliament votes to let terminally ill patients end their lives, the Archbishop of Canterbury warned, as he urged MPs to reject plans to allow assisted suicide.

The Most Rev Justin Welby and heads of Christian, Jewish, Muslim and Sikh groups have issued a joint letter to every MP asking them to oppose the assisted dying bill, which will be debated in the Commons on Friday.

The bill would allow doctors to prescribe a lethal dose to terminally-ill patients judged to have six months or less to live and who request it.

Currently, assisted suicide is illegal under the Suicide Act 1961 and is punishable by up to 14 years in prison.

Writing in the Observer newspaper, the archbishop said he and other faith leaders had spoken out because they believe changing the law would have “detrimental effects” on society.

“A change in the law to permit assisted suicide would cross a fundamental legal and ethical Rubicon,” he said.

“This respect for the lives of others goes to the heart of both our criminal and human rights laws and ought not to be abandoned.

“While it is not a crime in the UK for someone to take his or her own life, we recognise that it is a tragedy and we, rightly, do all that we can to prevent suicide.

“The assisted dying bill requires us to turn this stance on its head, not merely legitimising suicide, but actively supporting it.”

Mr Welby said asking to sanction doctors to take part in a person's bid to end their own life would be “a change of monumental proportions both in the law and in the role of doctors”.

He added: “Once a law permitting assisted suicide is in place there can be no effective safeguard against this worry, never mind the much more insidious pressure that could come from a very small minority of unsupportive relatives who wish not to be burdened.”

Among the faith leaders to sign the joint letter are Dr Shuja Shafi, secretary general of the Muslim Council of Britain, Ephraim Mirvis, chief rabbi of the United Hebrew Congregations of the Commonwealth and Lord Singh of Wimbledon, director of the Network of Sikh Organisations UK.

At least 35 Britons went to the Dignitas clinic in Switzerland to end their lives last year, campaign group Dignity in Dying said.

Sir Keir Starmer, a former director of public prosecutions, said people must be helped to take their own lives without being forced to travel to Switzerland.

The Labour MP said last week it was time politicians protected compassionate loved ones from prosecution by legalising assisted dying.

A fresh Commons bid to give terminally-ill patients the right to die will be based on legislation championed by former lord chancellor, Lord Falconer of Thoroton, which ran out of time in the House of Lords.

Campaigners hope the new Bill will give MPs the chance to consider a change in the law.

The measure will be steered through the Commons by Labour MP Rob Marris who topped the ballot for private members' bills, guaranteeing that the legislation will be debated.

Mr Marris said it could give MPs the first vote on the issue since 1997.