Suella Braverman has launched an astonishing personal attack on Rishi Sunak, describing the prime minister as weak and dishonest and claiming he reneged on promises to push through a series of controversial policy pledges. In a brutal three-page letter published a day after she was sacked as home secretary, Braverman warned Sunak that she now intends to spearhead a Tory rebellion over the government’s Rwanda plan. “Someone needs to be honest: your plan is not working, we have endured record election defeats, your resets have failed and we are running out of time. You need to change course urgently,” she wrote. Dozens of Conservative MPs are poised to demand that the government quits the European convention on human rights, a move resisted by senior cabinet ministers, if the UK’s highest court rules against sending asylum seekers to Rwanda on Wednesday. Home Office insiders said the government had no plan B in the event of losing in the supreme court, suggesting that ministers were “panicked” over the potential outcome. They warned it was highly unlikely that flights to Rwanda would be able to take off before February even if they won. In her letter, Braverman said that Sunak had failed to prepare a credible backup plan and had ignored her suggestion – believed to be emergency legislation to change domestic law so that the government could ignore the ruling and the flights could go ahead. “I can only surmise that this is because you have no appetite for doing what is necessary, and therefore no real intention of fulfilling your pledge to the British people,” she added. Sunak vowed to continue working to tackle small boat crossings regardless of the supreme court verdict. No 10 advisers believe that only a dozen or so backbenchers are prepared to fall in behind Braverman, while some Tory MPs feel the viciousness of her attack will make it harder for them to publicly back her. The prime minister has also been accused by a group of “red wall” and rightwing Conservatives of abandoning the voters who brought the party to power in 2019, as anger among some backbenchers grew over Braverman’s sacking and the surprise return of David Cameron. In her letter, Braverman claimed that Sunak had agreed to a secret pact to introduce key measures to secure her backing during the Tory leadership contest in October 2022, against Boris Johnson, but then “betrayed” the country by failing to deliver. These included policies to cut legal immigration, override the European convention on human rights (ECHR) to stop small boats crossing the Channel, deliver key post-Brexit laws and toughen up guidance for schools on transgender issues. “Despite you having been rejected by a majority of party members during the summer leadership contest and thus having no personal mandate to be prime minister, I agreed to support you because of the firm assurances you gave me on key policy priorities,” she wrote. She added: “This was a document with clear terms to which you agreed in October 2022 during your second leadership campaign. I trusted you … Our deal was no mere promise over dinner, to be discarded when convenient and denied when challenged. “You have manifestly and repeatedly failed to deliver on every single one of these key policies. Either your distinctive style of government means you are incapable of doing so. Or, as I must surely conclude now, you never had any intention of keeping your promises.” Sources close to Braverman claimed the agreement, which came after Liz Truss was forced out of No 10 – and when Sunak was fighting off a Johnson comeback – had been witnessed by others and that Sunak had taken a copy of the document away with him. After winning the Tory leadership contest when Johnson pulled out, Sunak stood outside Downing Street and promised the nation a government of “integrity, professionalism and accountability”. His former home secretary’s claims of a secret pact appear to undermine that pledge. Braverman claimed Sunak was likely to fail on his pledge to “stop the boats” by the end of the year regardless of the Rwanda ruling, and if the government lost then he would have “wasted a year” on the Illegal Migration Act “only to arrive back to square one”. Hard-right Tory factions are due to hold a crunch meeting 30 minutes after the supreme court releases its verdict to decide whether to call to leave the ECHR. If the ruling goes against the government, they plan to seek legal advice before releasing a statement which could put them on a collision course with Sunak. Even if the ruling allows asylum seekers to be deported to Rwanda, at least two MPs from the rightwing New Conservatives faction – Danny Kruger and Miriam Cates – have already called for the UK to leave the ECHR. A third, Sir John Hayes, a close ally of Braverman, suggested that in the event of losing, ministers should table a narrow piece of legislation to enact the Rwanda plan before Christmas, and later include withdrawing from the ECHR in the Tory election manifesto. Sunak is expected to argue in favour of remaining in the ECHR amid fears that leaving it could damage relations with Joe Biden. Under the Good Friday agreement, the ECHR underpins human rights guarantees in Northern Ireland. It also helps ensure judicial and legal cooperation with the EU under the terms of the Brexit deal. Braverman was sacked on Monday after opponents accused her of stoking tensions before pro-Palestine marches in London, days after claiming that police had applied a “double standard” to protesters in a newspaper article. In her letter, she criticised Sunak’s response to the marches as “uncertain, weak and lacking in the qualities of leadership that this country needs”, accusing him of “putting off” tough decisions to ban what she described as “hate marches” and tackle rising racism. A No 10 spokesperson said: “This government has brought forward the toughest legislation to tackle illegal migration this country has seen and has subsequently reduced the number of boat crossings by a third this year. And whatever the outcome of the supreme court tomorrow, he will continue that work.”