The Foreign Secretary deepened Cabinet divisions over possible post-Brexit customs arrangements by urging the Prime Minister to abandon one of the options under consideration.
Mr Johnson said a customs partnership scheme would not see the UK “taking back control” of its trade policy, laws, borders or money after Brexit, raising suggestions he could resign from Government if his objections are ignored.
The Government is still yet to decide between two options on future customs arrangements with the EU, which continue to split Mrs May’s Brexit “war Cabinet”.
The customs partnership scheme, believed to be favoured by the Prime Minister and backed by Chancellor Philip Hammond, would see the UK collect EU tariffs for goods coming into Britain on behalf of Brussels.
It is argued this would allow goods to pass freely between the EU and UK and prevent a hard border on the island of Ireland.
However, Brexiteers fear the plan would hinder the UK’s chances of being able to pursue an independent trade policy once outside the EU.
They favour a maximum facilitation, or “max fac”, scheme that would use technology and a “trusted trader” plan to reduce post-Brexit customs checks.
Signalling Brussels have rejected both options, Brexit Secretary David Davis recently admitted the EU have “pushed back” on the different UK proposals.
The Prime Minister’s official spokesman insisted on Tuesday Mrs May still had “full confidence” in Mr Johnson.
However, Downing Street also pointed out the entire Cabinet previously signed up to the two proposals outlined by the Prime Minister in her speeches in Florence and at Mansion House.
On the day of the latter, Mr Johnson posted a photo of himself holding a copy of the speech and giving a thumbs up.
Mrs May’s spokesman said: “There are two customs models that were put forward by the Government last August and most recently outlined in the Prime Minister’s Mansion House speech which the entire Cabinet was signed up to.
“Following last week’s sub-committee meeting, it was agreed that there are unresolved issues in relation to both models and that further work is needed.
“The Prime Minister asked officials to take forward that work as a priority.”
In his latest major public intervention over Brexit, Mr Johnson told the Daily Mail that a customs partnership is “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.
“If the EU decides to impose punitive tariffs on something the UK wants to bring in cheaply there’s nothing you can do.
“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.”
The Cabinet only briefly discussed Brexit on Tuesday morning, while the Prime Minister is reported to have delayed further discussions of the two customs options by her Brexit “war Cabinet” until next week.
Along with Mr Johnson, Mr Davis, Environment Secretary Michael Gove, International Trade Secretary Liam Fox, Defence Secretary Gavin Williamson and Home Secretary Sajid Javid are said to have opposed plans for a customs partnership at last week’s meeting of the Cabinet sub-committee.
However, the Daily Mail reported Mr Williamson, who backed Remain at the EU referendum, could be persuaded to change his mind.
Currently, there are said to be six ministers set against a customs partnership on the Cabinet sub-committee, while five back the proposal.
Cabinet opponents of the customs partnership scheme are backed by the influential European Research Group of Conservative backbench MPs.
Jacob Rees-Mogg, the ERG chair, has branded the proposal “just membership of the customs union and the single market by another name” and said Mr Johnson’s criticism “hits the nail on the head”.
Former Tory leader and fellow Brexiteer Iain Duncan Smith told Sky News: “We were led to believe last week that that was it – and they were going with the maximum facilitation process, which was also the Government’s plan.
“But, it turns out over the weekend, somehow, it’s been raised from the dead.”
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Mr Duncan Smith also insisted an eventual decision would not be the Prime Minister’s, adding: “We run a Cabinet government.”
At the weekend, Business Secretary Greg Clark, a supporter of the customs partnership, warned thousands of jobs in the car industry could be lost if there was not “a customs agreement that has the minimum of frictions”.