Boris Johnson has come under fire from both Downing Street and from Tory Remainers after he attacked Theresa May’s post-Brexit customs plans as “crazy”. 

No.10 reprimanded the Foreign Secretary for his remarks, as leading rebel Dominic Grieve warned that “overwhelming numbers” of MPs backed a soft Brexit and signalled they would vote accordingly in Parliament.

Cabinet divisions over the issue burst into the open after Johnson told the Daily Mail that May’s “customs partnership” proposal – where the UK would collect tariffs on behalf of the EU – would betray the promise to Leave voters to make Britain independent of Brussels.

Johnson had warned the idea would involve “a crazy system” of customs checks. 

“That’s not taking back control of your trade policy, it’s not taking back control of your laws, it’s not taking back control of your borders and it’s actually not taking back control of your money either, because tariffs would get paid centrally back to Brussels,” he told the paper.

Grieve added that the “rather extraordinary bursts of misbehaviour from Boris” proved that “he’s not inhibited by normal propriety in government”.

But after a string of Government defeats in the House of Lords on a ‘soft Brexit’ alternatives such as staying in a customs union or single market, Grieve warned that the Commons was set to be similarly robust.

With a wafer-thin majority propped up by the DUP, just 10 or so rebel MPs are needed to force May to change her plans.

In his strongest remarks to date, Grieve told BBC Radio 4′s World at One that Johnson’s “ideological” approach to Brexit had been laid bare by the way he breached usual rules on Cabinet collective responsibility.

With May facing a series of crunch Commons votes MPs on the EU (Withdrawal) Bill this month, Grieve warned: “I’m going to put country before party on this matter”.

“Boris’s comments worry me particularly in this regard,” Grieve said.

“I think you will find there are an overwhelming number of Members of Parliament who think that a continuing relationship with the EU facilitating frictionless trade is absolutely essential for our economic interests. And he doesn’t appear to take that view.”

Downing Street insisted that the PM retained “full confidence” in her Foreign Secretary, but its irritation was clear.

It pointed out that the customs partnership had been in existence for months, along with an alternative plan for a hi-tech streamlined customs arrangement.

“There are two customs models that were first put forward by the government last August and most recently they were outlined in the Prime Minister’s Mansion House speech – which the entire Cabinet was signed up to,” the PM’s official spokesman said.

“Following last week’s Cabinet sub-committee meeting it was agreed that there are unresolved issues in relation to both models and that further work is needed – as a priority.”

Others pointed out that Johnson had appeared to change his mind about the customs partnership idea that he had warmly endorsed as an option in the PM’s speech in March.

Lib Dem MP and Best for Britain campaign backer Layla Moran said: “With Boris’ latest outburst, today is the day that collective responsibility died.  

“Boris signed off these position papers as part of the Cabinet, yet today he has trashed it on the front page of a national newspaper.  How can he continue to sit in Cabinet.  It is scandalous.  Boris is a snake-oil salesman.”

Earlier this year, Johnson refused to rule out quitting the Cabinet if he felt Leave voters were betrayed.

The customs issue was not raised at the Cabinet meeting on Tuesday, but No.10 refused to deny that May had personally spoken to Johnson about his Daily Mail remarks.

In stark contrast to Johnson’s ‘crazy’ verdict, the spokesman said: “We believe there are two viable options”.

Some former Remain ministers believe that the deadlock in the Cabinet’s Brexit sub-committee can be bypassed if the full Cabinet – where hardline Brexiteers are outnumbered – could have the final say. 

Asked about putting the issue to the Cabinet, the PM’s spokesman said: “I’m not going to get into future discussions.

“What I can point you backwards towards is that in relation to speeches which the Prime Minister gave in Mansion House and also in Florence there was a full Cabinet discussion.”

Asked about Business Secretary Greg Clark’s strong defence of the customs partnership this weekend, stressing the importance of protecting UK jobs, the spokesman said his remarks were “entirely consistent” with the Government’s approach.

But when asked if Johnson was being entirely consistent, the spokesman sidestepped the issue.