BORIS Johnson has drawn a resignation line in the sand after dismissing Theresa May’s preferred Brexit customs plan as “crazy”.

No 10 suggested Britain’s withdrawal from the EU had not been raised at the weekly Cabinet as the Prime Minister and her Foreign Secretary sat across the same table. Mrs May’s spokesman, when asked if the two had had a discussion about Mr Johnson’s latest “tantrum”, as one ministerial aide put it, merely said: “The PM speaks to her Cabinet colleagues all the time.”

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But it appears the Foreign Secretary has made it abundantly clear that if the PM seeks to push through her preferred option of a customs partnership, he will walk.

He told The Daily Mail: “The plan would not comply with promises to take back control and would hamper the UK's ability to strike trade deals. It's totally untried and would make it very, very difficult to do free trade deals.

"If you have the new customs partnership, you have a crazy system whereby you end up collecting the tariffs on behalf of the EU at the UK frontier," he added.

Fellow Brexiteers Michael Gove, the Environment Secretary, and backbencher Jacob Rees-Mogg have described the customs partnership option as “bonkers” and “cretinous” respectively.

But pro-EU Tory MPs rallied around the embattled PM.

One told The Herald: “It’s not particularly helpful for the Foreign Secretary to directly contradict the PM on her preferred option and seek to undermine her. The Cabinet sub-committee should have their discussions in private.

“Boris would like to be sacked because it would make him a Brexit martyr and the PM has probably concluded it’s better to have him in the tent than outside it,” he added.

Dominic Grieve, the former Conservative Attorney General Dominic, said Mr Johnson's decision to speak out was "regrettable" but noted how he understood why Mrs May had put up with “these rather extraordinary bursts of misbehaviour by Boris".

No 10 insisted the PM still had full confidence in her Foreign Secretary but when her spokesman was pressed on the wisdom of him speaking out in public on such a sensitive issue, which Cabinet colleagues were still wrangling with, he dodged the questions.

Asked if the PM was happy for Mr Johnson to describe her preferred customs partnership option as “crazy”, the spokesman replied: “Look, there are two customs models that were first put forward by the Government last August and most recently they were outlined in the PM’s Mansion House speech, which the entire Cabinet was signed up to.

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

The spokesman insisted Mrs May was confident she could come up with an option that secured the support of all her Cabinet colleagues but declined to answer if she might seek a vote of the full Cabinet on the issue not just the 11 members of the sub-committee.

He also suggested the issue of Cabinet collective responsibility was not an issue. This, it is thought, is because the Government has not yet come up with a clear policy in choosing between the customs partnership option and the alternative called maximum facilitation, which the Brexiteers prefer.

At the weekend, Greg Clark, the Business Secretary, talked up the customs partnership option, saying as frictionless a border as possible was needed to protect jobs.

Meanwhile, Sir Vince Cable, the Liberal Democrat leader, said the public spat between the Government’s two warring factions made clear the depth of the Cabinet’s internal divisions on Brexit and made it less likely the Government would be taken seriously by Michel Barnier, the EU’s chief negotiator.

"Yet again this Tory Government is demonstrating to the Europeans who are negotiating with them that they are internally divided.

"When a leading Tory describes the Prime Minister's proposals as "cretinous", why would Mr Barnier take them seriously?" added Sir Vince.