Chancellor Philip Hammond has launched a scathing attack on Boris Johnson, dismissing the former foreign secretary's Brexit proposals as "fantasy world" and repeatedly saying he does not expect him to become prime minister.
After a day in which Theresa May and senior Tories lined up to heap criticism on her most high-profile critic, the Chancellor mounted a sustained assault on his former Cabinet colleague in a newspaper interview and a series of broadcast appearances.
Asked by the Daily Mail whether Mr Johnson could become prime minister, Mr Hammond said: "I don't expect it to happen," and suggested Mr Johnson could not do "grown-up politics".
He went on to attack the flamboyant Brexiteer for having "no grasp of detail" on complex subjects like Brexit, suggesting his greatest achievement to date had been introducing the "Boris Bike" cycle scheme while London mayor.
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The attack came at the end of the first day of the Conservative Party's annual conference in Birmingham in which its fault lines over Brexit were exposed with just weeks to go to settle a withdrawal deal with Brussels.
Mr Johnson had used a Sunday Times interview to describe Mrs May's Brexit policy as "deranged" and "preposterous".
In remarks that fuelled speculation about his leadership ambitions, the man who spearheaded the Leave campaign contrasted his position on Brexit with that of Mrs May, who backed Remain, saying: "Unlike the Prime Minister, I fought for this."
Mr Johnson is not speaking from the stage at this year's conference, after walking out of Cabinet in July in protest at the plan agreed at Chequers for the UK's future relationship with the EU.
But his scheduled speech on the fringe of the gathering on Tuesday is the most hotly-anticipated event of the four-day conference, with widespread expectations he will use it to step up his assault on the PM's plans.
In a round of broadcast interviews on Monday, Mr Hammond was repeatedly asked whether Mr Johnson could ever become prime minister, and stated several times: "I don't believe that will happen."
He told Sky News: "Of course, Boris is a big personality, nobody is denying that. What I'm saying is that the business of government is a process of attention to detail, follow-through, lots of hard work.
"It isn't just about making flamboyant statements and big announcements, it's about getting things done."
He dismissed the former foreign secretary's call for the UK to negotiate a "super-Canada" free trade agreement with the EU.
"It isn't about taking back control, it's about fantasy world," Mr Hammond told ITV's Good Morning Britain, arguing that the EU had made clear that a Canadian-style FTA covering the whole UK was not on the table, as Northern Ireland could not be included.
"When you go into a negotiation you have to understand the position of the people you're negotiating with," the Chancellor told BBC Radio 4's Today. "It's no good just ignoring it and banging your head against a brick wall.
"You have got to understand what their red lines are as well so you can try and find a landing ground you can both accept, which means a deal gets done."
Mr Hammond, who supported Remain in the 2016 referendum, insisted that he believes in Brexit and thinks there is "a high chance" that a version of the Chequers plan will be agreed. He told Radio 5 Live he was "not having sleepless nights" over the risk of a no-deal Brexit.
But he acknowledged the UK economy has suffered as a result of the vote to leave the EU, telling BBC1's Breakfast: "Clearly there has been a hit to our economy through the uncertainty that the Brexit process has caused.
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"Many businesses are sitting on their hands, frankly, waiting to see what the outturn of this negotiation is before confirming their investment plans, and of course that has an impact on the British economy."
But he added: "I believe that when the Prime Minister lands this deal and brings it back, there will actually be a boost to the economy, as businesses start making those investments that they've deferred over the last year or so, consumers start spending on big-ticket items as they feel more confident knowing where we are going in the future."
Foreign Secretary Jeremy Hunt used a Telegraph interview to warn Brussels it would stir up a "Dunkirk Spirit" if it forced Mrs May into a bad deal.
He told the paper: "If President Macron thinks... we will come crawling back desperate to rejoin the club in a few years' time... it is a profound misreading of our character."