Suella Braverman has accused Rishi Sunak of “betrayal” over a promise to stop small boat crossings in an incendiary letter after being sacked as home secretary.

In a broadside aimed at the Prime Minister, she accused him of having “manifestly and repeatedly failed to deliver” on key policies, and said his “distinctive style of government means you are incapable of doing so”.

She said Mr Sunak had not lived up to his promise to do “whatever it takes” to stop small boat crossings by failing to override human rights concerns about the Rwanda plan.

Mrs Braverman, one of the leading figures on the right of the Tory party, urged Mr Sunak to “change course urgently”, telling him he has led the Conservatives to “record election defeats” and that his “resets have failed and we are running out of time”.

Mr Sunak became Tory leader in the aftermath of Liz Truss’s disastrous and short-lived tenure without facing a vote of party members.

Mrs Braverman indicated she did a deal with him to secure her “pivotal” support – which she claimed he reneged on.

She pointed out he was “rejected by a majority of party members” in the contest with Ms Truss and had “no personal mandate to be Prime Minister”.

But she agreed to support him because of “firm assurances” on cutting legal migration, inserting measures to override the European Convention on Human Rights (ECHR) and the Human Rights Act on legislation to stop the boats, key pieces of Brexit legislation and the promise of “unequivocal” guidance to schools on protecting biological sex and safeguarding single-sex spaces.

“You have manifestly and repeatedly failed to deliver on every single one of these key policies,” she said, adding: “I must surely conclude now, you never had any intention of keeping your promises.”

Mrs Braverman said that if Mr Sunak did not agree to leave the ECHR he had to be prepared to “block off” the risk of human rights challenges to measures to curb migrant crossings.

“Your rejection of this path was not merely a betrayal of our agreement, but a betrayal of your promise to the nation that you would do ‘whatever it takes’ to stop the boats,” she said.

Ahead of Wednesday’s Supreme Court ruling on the Rwanda migration plan, Mrs Braverman accused Mr Sunak of “magical thinking – believing that you can will your way through this without upsetting polite opinion” and of failing to prepare a plan B should the justices rule against the Government.

But even if the Government succeeds and the Rwanda plan is backed, Mrs Braverman said the “compromises” made by Mr Sunak over the small boats measures will mean a “struggle to deliver” it in the way the public expects.

Mr Sunak sacked Mrs Braverman over the phone on Monday morning, clearing the way for a reshuffle which saw former prime minister David Cameron brought back into government as Foreign Secretary.

Mrs Braverman said being fired was “disappointing” but “for the best” as she aimed a series of blows at Mr Sunak.

The nail in her ministerial coffin may have been an unauthorised article for The Times in which she accused police of “double standards” because of the way pro-Palestinian “mobs” have been handled.

She said she pushed Mr Sunak to legislate to “ban the hate marches”, saying “Britain is at a turning point in our history and faces a threat of radicalisation and extremism in a way not seen for 20 years”.

But she told him: “I regret to say that your response has been uncertain, weak and lacking in the qualities of leadership that this country needs.”

Mr Sunak was already facing anger from the right of the Tory party.

Miriam Cates and Danny Kruger, who co-chair the New Conservatives grouping of MPs, stressed their support for the Prime Minister, but expressed deep disappointment that Downing Street had decided to give up on the voters Boris Johnson won over in the 2019 general election.

The two MPs said Mr Sunak’s reshuffle means the party is now “sacrificing” the swathes of seats in the “red wall” of former Labour heartlands it won four years ago.

They said: “Until yesterday, we held onto the hope that the Government still believed in the realignment – that they would work to rebalance our economy, reorient our foreign policy, radically reduce migration, and restore common sense in our schools and universities. That hope – the project of the realignment – has now dwindled.

“In political terms, it appears the leadership has decided to abandon the voters who switched to us last time, sacrificing the seats we won from Labour in 2019 in the hope of shoring up support elsewhere.”

Mr Sunak held his first post-reshuffle Cabinet meeting on Tuesday morning, hailing his “strong and united team”.

New polling from Ipsos UK suggests that 70% of people backed Mr Sunak’s decision to sack Mrs Braverman, as the Prime Minister hopes a pivot away from her more polarising rhetoric can help restore his electoral fortunes.

He faces a crunch week ahead, with new inflation figures and the Supreme Court’s Rwanda ruling on Wednesday.

Chancellor Jeremy Hunt will also deliver his autumn statement next week under pressure from some Tories to unveil significant tax cuts.