In an incendiary letter a day after she was sacked as home secretary, Ms Braverman told the prime minister he had “manifestly and repeatedly failed to deliver” on key policies.
And she hit out at Mr Sunak for what she said was a “betrayal” of his pledge to do “whatever it takes” to stop migrants arriving on boats in return for her support during the Tory leadership contest.
In an extraordinary move she also:
Claimed the Tory leader “never had any intention” of keeping promises and of defying her wish to override the European Court of Human Rights on Rwanda
Ripped into his “weak” response to what she called “hate marches” and said someone had to be “honest” about his failures
Urged Mr Sunak to “change course urgently” – sticking the boot in to remind him that he had led the Tories to “record election defeats”
Revealed she had a “document” setting out what she said was their private agreement
Warned that with an election looming “your resets have failed and we are running out of time”
Said she would support “authentic Conservative” government policies, in what will be seen as a leadership pitch
Ahead of a landmark Supreme Court decision on the Rwanda policy, Ms Braverman also warned that even if the policy gets the green light, it will not work in the way the public expects.
Challenges could last “months” and his “compromises” will mean the asylum policy could be “thwarted yet again” by the European Court of Human Rights.
Justices in the UK’s highest court are set to rule on whether or not the scheme is unlawful, following a Court of Appeal ruling from June that overturned the High Court’s finding that Rwanda could be considered a “safe third country” for migrants.
The sacked home secretary claimed she had posed her own “credible” back-up – without which she said there is “no hope of flights this side of an election” in the event of a court defeat – but received no reply.
“Worse than this, your magical thinking – believing that you can will your way through this without upsetting polite opinion – has meant you have failed to prepare any sort of credible ‘Plan B’,” she wrote.
Ms Braverman also accused the prime minister of “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”.
The furious ex-home secretary also alluded to a deal she struck with Mr Sunak in the leadership contest. At the time her backing was seen as crucial to his campaign.
She said she had agreed to return as home secretary – after she was sacked by Liz Truss – on “certain conditions”, including taking action on the effect of the European Convention on Human Rights (ECHR) on small boats legislation.
That agreement was a “document with clear terms”, she said, adding: “I trusted you.” But she accused Mr Sunak of “equivocation, disregard and a lack of interest”.
Her exit from the cabinet followed widespread outrage over her accusation that the Metropolitan Police was biased and “playing favourites” with pro-Palestine protesters.
On Tuesday afternoon, a No 10 spokesperson said Mr Sunak would continue working to tackle small boat crossings regardless of the court ruling.
“The prime minister believes in actions not words,” they said. “He is proud that 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.”
In her letter, Ms Braverman also lashed out at Mr Sunak over the protest – accusing him of being “uncertain, weak and lacking in the qualities of leadership that this country needs”.
She claimed she had “become hoarse urging you to consider legislation to ban the hate marches and help stem the rising tide of racism, intimidation and terrorist glorification”.
Earlier on Tuesday, Mr Sunak dumped Ms Braverman’s controversial plan to restrict tents being given to homeless people. No 10 said it would not be included in the Criminal Justice Bill.
Ms Braverman had sparked outrage with the so-called tents ban and her description of rough sleeping as a “lifestyle choice”.
In her letter, Ms Braverman wrote: “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.”
She added: “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.”
Mr Sunak will come under intense pressure from Tory MPs to pull out of the ECHR if judges rule against his Rwanda policy.
New home secretary Mr Cleverly said in April that he was “not convinced” the move is necessary.
But Jacob Rees-Mogg, the former business secretary, told The Independent the government had to “get flights going by the end of the year or the policy won’t be taken seriously”.
He urged Mr Sunak to “override” the ECHR, saying “Tory MPs … will expect the PM to deliver.”
Conservative MP Martin Vickers – a member of the Common Sense Group of Tory right-wingers – said Mr Sunak would come under “considerable pressure” to quit the ECHR if he loses the Rwanda case and to "make a clean break".
In its reply to her letter, Downing Street thanked Mrs Braverman for her service but did not touch on the suggestion of a backroom deal.
Liberal Democrat home affairs spokesman Alistair Carmichael spoke of “yet more Conservative chaos ... When will this Conservative Party soap opera end?”