Suella Braverman has delivered a scathing attack on Rishi Sunak, accusing him of failing to keep his promises over immigration.
In a blistering letter to the prime minister, the former home secretary told him “your plan is not working”.
She said Mr Sunak had betrayed his pledge to do “whatever it takes” to stop small boats crossing the Channel.
Mrs Braverman, a leading figure on the right of the party, was sacked as home secretary on Monday.
Her sacking triggered a major cabinet reshuffle, which saw former Prime Minister David Cameron return to government as foreign secretary.
A No 10 spokesman thanked Mrs Braverman for her service, but added: “The prime minister was proud to appoint a strong, united team yesterday focused on delivering for the British people.”
In her letter, the former home secretary told Mr Sunak he had “manifestly and repeatedly” failed to deliver on policy priorities.
“Either your distinctive style of government means you are incapable of doing so,” she wrote.
“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.”
Mrs Braverman was sacked from her role, after opponents accused her of stoking tensions ahead of pro-Palestinian marches in London.
She lost her job days after she claimed police had applied a “double standard” to protesters, in an article for the Times newspaper.
Mrs Braverman said Mr Sunak had failed “to rise to the challenge posed by the increasingly vicious antisemitism and extremism displayed on our streets”.
Mrs Braverman told the PM that if the ruling went against the government, he would have “wasted a year” on the Illegal Migration Act, which aimed to stop small boat crossings, “only to arrive back at square one”.
Labour minister Lisa Nandy said the letter was “just the latest instalment in a Tory psychodrama that’s been playing out over the last 13 years, holding the rest of the country to ransom while the Tories fight among themselves”.
