Suella Braverman has accused Rishi Sunak of betraying the nation by backtracking on a secret deal to ignore European Convention on Human Rights (ECHR) rulings.

In one of the most scathing departure letters from a Cabinet minister in recent history, the former home secretary claimed the Prime Minister had dodged “hard decisions” on how to “stop the boats”.

She said Mr Sunak’s approach to tackling illegal migration across the Channel had seen him use “wishful thinking as a comfort blanket” and amounted to “irresponsibility”.

Mrs Braverman condemned his attempts to make the Tories appealing to voters and demanded a return to an “authentic conservative agenda”, in what some may see as a threat of a future leadership challenge.

Mrs Braverman claimed in her letter that legislating against the ECHR was one of four promises Mr Sunak made to her.

The other three were measures to bring down illegal migration, delivering the Northern Ireland Protocol and rewording of Retained EU Law Bills in their existing form, and issuing “unequivocal” guidance for schools to protect biological sex.

Mrs Braverman claimed that she pushed Mr Sunak at times with letters about the promises, but “this was often met with equivocation, disregard and a lack of interest”.

Her attack was published ahead of Wednesday’s Supreme Court ruling on whether the Government’s scheme of deporting migrants who arrive in the UK in small boats to Rwanda is legal.

She wrote: “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.”

And, she continued, “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.”

At its heart is a claim that Mr Sunak signed a secret deal to secure her backing for his Tory leadership bid to replace Liz Truss last autumn, in which he promised to ignore the ECHR if it stood in the way of stopping small boats carrying migrants across the Channel.

Mrs Braverman wrote: “I was clear from day one that if you did not wish to leave the ECHR, the way to securely and swiftly deliver our Rwanda partnership would be to block off the ECHR, the HRA [Human Rights Act] and any other obligations which inhibit our ability to remove those with no right to be in the UK. Our deal expressly referenced ‘notwithstanding clauses’ to that effect.”

Downing Street dismissed the three-page letter publicly, with a No 10 spokesman saying: “The Prime Minister was proud to appoint a strong, united team yesterday focused on delivering for the British people. The Prime Minister believes in actions, not words.”