Ministers are deliberately hiding the mounting costs of the Rwanda deportation scheme from the public, the head of an influential parliamentary watchdog has told the Guardian, as insiders expect a new deal with the African country to be signed off within days. Dame Diana Johnson, the chair of the home affairs select committee, said the government had “total disregard” for parliament’s rights to scrutinise the key immigration policy after a senior civil servant said that any extra costs on top of the £140m already paid to Rwanda would not be disclosed until the summer. The home secretary, James Cleverly, is expected to fly to Kigali early this week to sign a treaty with the Rwandan government to get around a supreme court ruling that the scheme was unlawful. Emergency legislation declaring the central African country to be safe and which aims to curb legal challenges against the policy could also be presented to parliament. The Sunday Times reported that the UK government would hand over another £15m on top of the £140m – a claim that was dismissed as “speculation” by the health secretary, Victoria Atkins. On Wednesday, Matthew Rycroft, the home office’s permanent secretary, told the select committee that any additional costs would be disclosed in the department’s annual update in the summer. “Ministers have decided that the way to update parliament is annually, rather than by giving a running commentary,” Rycroft said. Johnson, the Labour MP for Kingston upon Hull, said on Sunday that the apparent leak to the Sunday Times, days after the refusal to disclose figures to parliament, was “totally unacceptable”. She plans to raise the lack of policy transparency with the immigration minister, Robert Jenrick, on Wednesday afternoon, when he is due to appear before the committee. “Last Wednesday, the home affairs select committee was told that any extra spending on the Rwanda scheme, on top of the £140m already spent, will not be made public, or given to parliament, until next summer. “This was clearly a decision of ministers and information could be made available if they wished to do so. This approach makes effective in-year scrutiny of the department and its policies impossible. It is totally unacceptable that journalists are given this £15m figure now. It shows a total disregard for the vital role that select committees play in holding the government to account,” she said. A source close to the home secretary said: “We do not recognise the figure published nor how it was come up with. To be clear with the framing of that figure, the Rwandans have not asked for extra money in order to sign a treaty, nor has money to secure a treaty been offered.” The committee could ask the National Audit Office to conduct a short inquiry into the Rwanda scheme, or could recall Rycroft to answer further questions, it is understood. Sunak’s key immigration policy was dealt a severe blow last month after the UK’s highest court found there was a real risk of deported refugees having their claims wrongly assessed or being returned to their country of origin to face persecution. Since then, Cleverly’s senior officials have been drawing up a new deal with the Rwandan government in a bid to satisfy the court’s key criticisms. It is understood that there will be additional payments within the deal which will be described as contributions towards training and additional staff costs. A Home Office spokesperson said: “All costs will be reported as part of the annual Home Office reports and accounts in the usual way.” It comes amid heated discussions in government about how far the new legislation should go to sidestep future legal challenges. Backbench Conservative MPs from the hard right have claimed that the legislation will fail unless it also disapplies the European convention on human rights (ECHR). Meanwhile, MPs from the centre left of the party have warned that dozens could rebel if the government steps away from international conventions, joining pariah states such as Russia and Belarus. No 10 has been considering two options. The first, the so-called semi-skimmed option, would disapply only the UK’s Human Rights Act in asylum claims. However, this would not prevent challenges by individual migrants, sources said. The second, “full-fat” option would remove the right of judicial review and include “notwithstanding clauses”, which would allow ministers to ignore the ECHR and other international treaties in the area of asylum. It is understood that ministers are closing in on a “middle way” between the both options which will not be finalised until the treaty has been signed. Insiders said that Victoria Prentis, the attorney general, has been overseeing the legislation, while Jenrick has had a “minor role”. She is often characterised as reluctant to withdraw the UK from international treaties, while Jenrick has embraced many of the hardline policies put forward by Suella Braverman, the former home secretary, who was sacked last month. Refugee charities have said that they cannot see how the legislation would prevent legal challenges. They say the supreme court judgment was comprehensive and damning of the Rwandan government’s asylum system, and that legislation cannot take away the right of an individual to launch a legal challenge. The Rwanda proposal is seen as key to delivering on Rishi Sunak’s pledge to “stop the boats” before a general election, which could come as soon as May. Home Office figures show that 519 people braved freezing December temperatures to land on England’s south coast on Saturday, arriving in 11 boats. Almost 29,000 migrants have arrived via this route this year, topping 2021’s 28,526 figure.