Britain will cross a “legal and ethical Rubicon” if parliament votes to permit terminally ill patients to end their lives, said the archbishop of Canterbury, Justin Welby, as leaders of all the UK’s major faith groups call on MPs to reject plans to allow assisted suicide.

In an extraordinary show of unity on Sunday, the heads of Britain’s Christian, Jewish, Muslim and Sikh communities wrote a joint letter to every MP – published in the Observer – urging them to throw out the assisted dying bill, which will be debated in the Commons on Friday.

Why I believe assisting people to die would dehumanise our society for ever | Justin Welby Read more

The private member’s bill would allow patients judged as having no more than six months to live, and who had a “clear and settled intention” to end their lives, to be prescribed a lethal dose of drugs. Two doctors and a family court judge would have to assess the patient’s diagnosis and prognosis, and check that he or she was mentally competent to make a judgment, free of coercion. The patient would then have to administer the lethal medication themselves, with a healthcare professional present.

While few private members’ bills make it on to the statute book, this legislation, put forward by Rob Marris, the Labour MP for Wolverhampton South West, is almost identical to a bill advanced by former lord chancellor Lord Falconer, which passed its initial legislative stages in the upper house last year before running out of parliamentary time. As such, it is as seen as having a strong measure of parliamentary support before it enters the Commons.

But writing in the Observer, the archbishop says he and other faith leaders have decided to speak out – not to advance their religious viewpoint but because they believe the bill would have serious detrimental effects on individuals and society, and compromise the principle of respect for the lives of others that lies at the centre of the legal system.

“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,” writes Welby. “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. We are asked to sanction doctors participating in individuals taking steps to end their lives. This is a change of monumental proportions, both in the law and in the role of doctors; it is little wonder that it is opposed by the medical profession.”

Facebook Twitter Pinterest Portrait of the late Tony Nicklinson, with his wife Jane, who campaigned for assisted dying. Photograph: Stephen Shepherd/The Observer

Welby also argues that thousands of vulnerable people could be placed under pressure to end their lives prematurely. He cites concerns that in the US states of Oregon and Washington, where similar laws are in place, between 40% and 61% of those who administered drugs to end their lives had mentioned worries that they would be a burden on their families as a factor in their decision.

“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,” says Welby. “The exhaustion of caring, sometimes combined with relationships that have been difficult for years before someone fell ill, can lead people to want and feel things that they should not.

“All of us who have been involved in pastoral care and bereavement care have heard the confusion people feel about how they behaved to a demanding relative. The tests in the bill do not make space, and never could, for the infinite complexity of motives and desires that human beings feel. The law at present does make that space, and yet calls us to be the best we can.” He asks whether we really want to create a society “where each life is no longer seen as worth protecting, worth honouring, worth fighting for”.

The Observer view on assisted dying | Observer editorial Read more

All the main parties have suggested that their MPs will be given a free vote and that the issue will treated as one of conscience. If sufficient MPs vote to give the bill a second reading on Friday, the government will then have to decide whether or not to find parliamentary time to allow it a chance to go further.

Last year the supreme court raised pressure on MPs to act by suggesting that it had the power to declare that the current law, which makes it illegal for people to take their own lives, is “incompatible” with human rights.

This came after it turned down a challenge to the status quo brought by the widow of Tony Nicklinson, who suffered from “locked-in syndrome” and fought a long campaign for assisted suicide.

Assisting a suicide is an offence under the Suicide Act 1961 and is punishable with up to 14 years’ imprisonment. This includes accompanying someone abroad to the Swiss clinic Dignitas. However, guidelines published by the director of public prosecutions in 2010 effectively decriminalised it if the decision was made voluntarily and in a well-informed manner, and the person assisting was wholly motivated by compassion.

Assisted dying: leaders of faith communities speak out against new bill | the big issue Read more

Lord Falconer said on Saturday night: “No one disputes the law is a mess – the former DPP who tried to make it work, the police, the courts right up to the supreme court, and above all people who are dying and those who love them. Legislators and the government need to address the mess and find a solution. It is so clearly time for the government to step in and set up a trusted process which will seek a solution in which the public will have confidence.”

Marris, who is tabling the bill, said: “With strong safeguards, terminally ill adults of sound mind should be allowed a legal choice to have assistance to end their own lives. Those who believe that ending one’s own life is always wrong should not deny choice to those of us who do not share their beliefs.

“The current law is a mess, with home suicides, technically illegal actions by some doctors, and Dignitas deaths. DPPs have had to issue guidelines on when it is not in the public interest to prosecute – but in a democracy it is elected representatives who should make the law.”

Sarah Wootton, chief executive of Dignity in Dying, said: “The question is no longer whether the law is broken, as has recently been confirmed by the former director of public prosecutions, but rather whether MPs will fix it.”