Theresa May’s Brexit strategy has put the UK constitution in a “suicide vest” and handed the detonator to Brussels’ chief negotiator Michel Barnier, Boris Johnson says.

Britain’s former foreign secretary’s extraordinary comments provoked an immediate backlash from Tory critics in the latest sign of the bitter Conservative divide over Brexit and the future leadership of the party.

Mr Johnson launched the attack amid further focus on his private life following the announcement that he has separated from his wife Marina Wheeler and the couple are divorcing.

The prominent Brexiteer’s latest assault on the prime minister’s handling of negotiations with Brussels will fuel speculation about his own leadership ambitions.

British Prime Minister Theresa May returned to Parliament on September 5. Photo: Getty

Mr Johnson quit the Cabinet in opposition to May’s Chequers plan which would see the UK remain closely aligned with EU rules on goods.

Writing in the Mail On Sunday, he said: “At every stage in the talks so far, Brussels gets what Brussels wants.

“We have agreed to the EU’s timetable; we have agreed to hand over STG39 billion, for nothing in return.

“Under the Chequers proposal we are set to agree to accept their rules – forever – with no say on the making of those rules.

“It is a humiliation. We look like a seven-stone weakling being comically bent out of shape by a 500lb gorilla.”

He also lashed out at the Northern Ireland “backstop” – the measure aimed at making sure there is no hard border with Ireland.

Protesters have their say against Theresa May’s Chequers plan for Brexit outside the gates of Downing Street on September 5. Photo: Getty

Under the EU’s version of the plan, if no trade deal with the UK resolved the issue, Northern Ireland would effectively remain part of the single market.

Mr Johnson said: “We have opened ourselves to perpetual political blackmail. We have wrapped a suicide vest around the British constitution – and handed the detonator to Michel Barnier.

“We have given him a jemmy with which Brussels can choose at any time to crack apart the union between Great Britain and Northern Ireland.”

The UK’s alternative backstop and the Chequers plan would both mean “agreeing to take EU rules, with no say on those rules”, leaving the country a “vassal state”.

“We have managed to reduce the great British Brexit to two appalling options: either we must divide the Union, or the whole country must accept EU law forever,” Mr Johnson said.

-AAP