Suella Braverman accused Rishi Sunak of “magical thinking” for failing to come up with a back-up plan if his Rwanda policy is blocked in the Supreme Court and said he could betray his promise to “stop the boats” even in the event of victory.

In an excoriating letter to the Prime Minister after being sacked as home secretary, Mrs Braverman warned that Mr Sunak has no “credible Plan B” if justices in the UK’s highest court rule the scheme is unlawful on Wednesday.

She claimed that even if the policy gets the go-ahead then his “compromises” will mean the asylum policy could be “thwarted yet again” by the European Court of Human Rights.

Her replacement James Cleverly had outlined the possible outcomes during the first meeting of the Prime Minister’s new-look Cabinet after the dramatic reshuffle that saw Mrs Braverman shown the door.

Senior ministers have wargamed responses to a possible defeat of what Downing Street admits is a “crucial” policy but Mrs Braverman warned of a “betrayal” of Mr Sunak’s promise to do “whatever it takes” to stop unauthorised Channel crossings all the same.

The sacked home secretary wrote in the letter she published online that if they lose he will have “wasted a year” on the Illegal Migration Act “only to arrive back at square one”.

“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 said.

She said she 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.

Even in the event of Supreme Court victory, she said the Government “will struggle to deliver our Rwanda partnership in the way that the public expects”.

She said the Act is “far from secure against legal challenges” and will “leave us vulnerable to being thwarted yet again by” the European court in Strasbourg.

“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 accused Mr Sunak of having opted for “wishful thinking as a comfort blanket to avoid having to make hard choices” as he refused to consider leaving the European Convention on Human Rights, or even overruling elements in his legislation.

Downing Street vowed to continue to work to tackle small boat crossings “whatever the outcome” in the Supreme Court.

“The Prime Minister believes in actions, not words,” a No 10 spokeswoman said in response to Mrs Braverman’s declaration of political war.

Downing Street earlier said Mr Cleverly had briefed the Cabinet over “some of the possible scenarios” to Wednesday’s ruling.

Leaving the European Convention on Human Rights was not discussed at the meeting, the Prime Minister’s official spokesman said.

Contingency plans have been “discussed among Cabinet ministers”, the spokesman said, and “options for possible scenarios” have been prepared.

Last month, the Home Office challenged 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.

Lawyers representing people facing deportation to the east African nation argue Rwanda is an “authoritarian, one-party state” with a “woefully deficient” asylum system.

But the Home Office has said the policy to remove asylum seekers to a “country less attractive” than the UK, “but nevertheless safe”, is lawful.

Five justices at the Supreme Court will give their verdict on Wednesday morning, before Mr Sunak faces Prime Minister’s Questions.

The Illegal Migration Act brought into law the Government’s policy of sending some asylum seekers to Rwanda.

However, the plans announced in April 2022 have been held up in the courts, with no deportation flights having taken place despite £140 million already being handed to Kigali.

Whereas Mrs Braverman repeatedly signalled she wanted out of the “politicised court”, Mr Cleverly said while foreign secretary in April he was “not convinced” the move is necessary.

He said that the European countries that are not signatories – Russia and Belarus – are a “small club”, adding: “I am not convinced it is a club we want to be part of.”

Mr Sunak has set stopping small boats of asylum seekers from arriving in Britain as one of his five pledges to the electorate.

But more than 27,300 migrants have been detected making unauthorised crossings of the English Channel so far this year, according to official figures.

The Supreme Court ruling by Lords Reed, Hodge, Lloyd-Jones, Briggs and Sales will be handed down on Wednesday after 10am.