Boris Johnson has attacked Theresa’s May’s Brexit plan, saying she had “wrapped a suicide vest” around the British constitution and “handed the detonator” to Brussels.
Writing in the Mail on Sunday, the former foreign secretary said the Chequers deal had opened the UK to “perpetual political blackmail”.
Mr Johnson quit his post in July after the plan was agreed by the cabinet.
Some Tory MPs have strongly criticised his language in the article.
Writing in the same paper, the current Foreign Secretary Jeremy Hunt called on the country to “back” the prime minister and her plan.
It is the first public comment from Mr Johnson since he and his wife, Marina Wheeler, confirmed they were getting divorced earlier this week.
The announcement followed newspaper allegations the former London mayor had been having an extra-marital affair.
In 2004, Mr Johnson was sacked from a shadow ministerial role by then-Conservative leader Michael Howard for allegedly lying over an affair with journalist Petronella Wyatt – something he had denied as an “inverted pyramid of piffle”.
Elsewhere, the Sunday Times has reported that Mrs May’s aides wrote a 4,000-word “dirty dossier” on Mr Johnson in 2016, exposing a “catalogue of lurid allegations” against him.
The paper says it has seen the document – said to have been written during the Conservative leadership contest – but officials at Downing Street and Conservative Campaign Headquarters have denied circulating it.
In his newspaper article, Mr Johnson accused the EU of “bullying” the UK – but questioned why the response had been “so utterly feeble”.
He said that rather than getting a “generous free trade deal”, Britain is saying, “yes sir, no sir, three bags full sir”, to Brussels.
“At every stage of the talks so far, Brussels gets what Brussels wants,” wrote Mr Johnson.
“It is a humiliation. We look like a seven-stone weakling being comically bent out of shape by a 500lb gorilla.”
He said the reason for this was “simple”, namely Northern Ireland and the “insanity of the so-called backstop”.
The backstop solution is a safety net the UK and EU signed up to in December 2017, which seeks to maintain cross-border co-operation, support the all-island economy and protect the Good Friday peace agreement.
Mr Johnson said agreeing to this had “opened ourselves to perpetual political blackmail”.
He added: “We have wrapped a suicide vest around the British constitution – and handed the detonator to Michel Barnier [the EU’s chief negotiator].”
Mr Johnson wants the UK to pursue a technological solution for the border, but says the government has always had a “secret agenda” to keep the UK in the single market and customs union.
He concluded: “We have put our own heads deliberately on the block.”
The BBC’s political correspondent Alex Forsyth said the “scathing remarks” showed Mr Johnson’s “willingness to challenge the prime minister remains undimmed” after the recent revelations about his private life.
It is the former London mayor’s second attack on the prime minister’s Brexit strategy inside a week, as last Monday he used his Daily Telegraph column to say the Chequers deal “means disaster” for Britain.
Mr Johnson’s latest intervention has been criticised by two senior Conservative MPs.
Former Army officer and member of the Foreign Affairs Select Committee Tom Tugendhat graphically described the aftermath of a suicide bomb that had gone off near his office in Helmand province, Afghanistan.
He said comparing such an attack to the PM’s plans “isn’t funny”, adding: “Some need to grow up”.
Foreign Office Minister Sir Alan Duncan criticised his former boss, saying the article was “one of the most disgusting moments in modern British politics.”
He added that if this latest broadside was not the end of Mr Johnson’s political career, “I will make sure it is later”.
Mr Hunt expressed his support for Mrs May and the Chequers deal in the newspaper on the opposite page from Mr Johnson’s article.
He wrote that the PM is “better than anyone I know at holding the line in the face of intense pressure”.
But he called on the country to “unite behind her” to strengthen her position.
“Parliament will, of course, have the chance to debate and vote on any agreement,” wrote Mr Hunt.
“Until then, we should not rush to judgement on a deal that is still under negotiation.
He added: “However tough the negotiations, we must not and will not ignore the will of the British people.
“So this is the moment to back the prime minister who is determined to deliver on their instructions.”