Labour divisions over the controversial bill on assisted dying have intensified as party grandee Harriet Harman demanded the immediate cancellation of work ordered by the health secretary, Wes Streeting, into the potential costs to the NHS of legalisation. In an interview with the Observer, Harman said Streeting’s intervention – making clear he regarded the bill as a “slippery slope” and saying that he had commissioned work on the financial implications – had not only breached “neutrality” rules for cabinet ministers but risked “tainting” the entire debate by turning it into an issue of “pounds and pence”. Harman, who backs the private member’s bill introduced by Labour MP Kim Leadbeater, said: “I think it is really important that the government is neutral on this and the two people whose neutrality is most important are the prime minister and health secretary. Keir has stuck to that – but Wes has not.” Now in the House of Lords, Harman said she strongly supported Streeting on other issues, including his proposed changes to the NHS, but added: “He has already stepped over the line by saying how he is going to vote.” Now he needed to “cancel the work” he ordered on the potential costs. “By commissioning work to assess the cost of facilitating assisted dying – which he will have to publish – he will then of necessity have to balance that against the cost of the person staying alive. That leads you to the awful prospect that the research could find that it is cheaper for people to be doing assisted dying rather than staying alive, and that would really contaminate the argument,” said Harman, who is co-host of the podcast Electoral Dysfunction. “It has to be an argument about individual choice and moral principle. It cannot be an argument about money.” It was already clear from experience in other countries that the costs would be tiny in the context of the overall NHS budget, she argued. “I think he should not go ahead with this research because either way it is problematic, especially if it finds that it is cheaper for the NHS for people to have assisted dying. That will taint the decision with the idea that people who voted for it are voting for it to save money. “The most important thing is for him to cancel the work he has commissioned and henceforth… to say absolutely nothing. “This debate has to proceed not on the basis of pounds and pence. It has not to be a debate about money but about morals and practicality.” Only a few weeks ago it seemed highly likely that the bill would pass through parliament, albeit after intense debate both in the Commons and Lords. The fact that Keir Starmer, while in opposition, had voted in favour of assisted dying, and with Labour now enjoying a huge Commons majority, seemed to mean a historic change was on the cards. But ahead of the second reading of the bill on 29 November, the debate is intensifying. Other cabinet ministers have been taking sides, including justice secretary Shabana Mahmood, who has said she will not support it, and culture secretary Lisa Nandy and work and pensions secretary Liz Kendall, who have indicated they will back it. On Sunday, public figures opposed to the bill, including the paralympian Baroness Grey-Thompson, former Supreme Court justice Lord Sumption and former attorney general Dominic Grieve, will endorse a report by the thinktank Policy Exchange that comes out strongly against assisted dying. The report argues, from international experience, that legalising assisted dying for the terminally ill means it will then “be extended to many other patients, including the frail elderly and people with a range of disabilities”. It also says that the case advanced for legalisation unduly prioritises the argument for individual autonomy, holding this to be the paramount principle at the expense of other important considerations, such as the sanctity of life. Sumption writes: “The lesson of this paper is that medically assisted suicide arouses strong feelings, and strong feelings make for muddled thinking and moral confusion … It would be difficult to argue that the interventions of courts and judges have been either coherent or consistent. What is the justification for allowing medically assisted suicide but limiting it to those believed to be close to death or in intolerable pain, actual or prospective? There are so many other reasons why one might want to end one’s life. Once the moral barrier has been crossed, what is the logical stopping point?” Harman urged members who were in favour in principle of assisted dying but worried about a lack of safeguards, to vote for it on 29 November because ample time would be allowed in parliament to debate all issues thoroughly and make changes. “They should vote for it at second reading and then the safeguards will be scrutinised in committee – and if they are still not happy with the safeguards they can vote against it at third reading … This is really important because some people are saying: ‘I am happy with the principle but I am worried about the safeguards’. “This is the biggest piece of liberalising social policy change in a generation, up there with the abolition of capital punishment and the allowing of abortion,” Harman said. She added: “This is a very big moment for MPs and peers.”