Boris Johnson “threatening Theresa May from thousands of miles away”

There was dismay among Tory Hard Brexiteers at reports that Mrs May’s Brexit war cabinet had discussed a plan under which the whole of Britain would observe EU external tariffs until practical arrangements are in place to keep the Irish border open.

Mr Johnson last night called on the prime minister “true to her word” and deliver a final settlement in which the UK was not tied to the EU’s system of common external tariffs.

Speaking to reporters in Buenos Aires, Argentina, the foreign secretary said: “Brexiteers fearing betrayal over the customs backstop must understand that the PM has been very clear that neither option is an outcome we desire – we want a deal with the EU and she will deliver it.

“I’m convinced that the prime minister will be true to her promises of a Brexit deal – that sees Britain come out of the customs union and single market, have borders as frictionless as possible, reject European Court of Justice interference, control immigration and free to conduct unhindered free trade deals across the world.

“We must now give the prime minister time and space to negotiate this Brexit vision.”

Tom Brake, Liberal Democrat MP and champion for the pro-EU group Best For Britain described Mr Johnson’s words as a “thinly-veiled Brexit threat”.

He said: ‘It doesn’t matter that he is halfway round the world, Boris can still be relied upon to put the boot in to the prime minister. “His disloyalty isn’t diminished by distance. He is implacably opposed to her ‘crazy’ customs partnership solution. “The only problem for him is that his ‘max fac’ border solution doesn’t stack up either. He and the PM are just as clueless. This is why the people need a final say on the deal.’

There are fears among Tory Hard Brexiteers that UK involvement in EU structures beyond the conclusion of the transition period in December 2020 may end up being indefinitely extended.

The foreign secretary is understood to be one of two ministers who argued against the backstop option in last week’s meeting of the Cabinet sub-committee overseeing Brexit strategy and negotiations.

The scheme was devised as an alternative to the European Commission proposal that Northern Ireland should remain in the customs union if no better resolution for the border issue could be found.

This Brussels backstop was roundly rejected by Mrs May as something no British PM could sign up to, as it effectively draws a customs border down the Irish Sea.

While he regards the new proposal as preferable to the Commission’s scheme, Mr Johnson left no doubt he believes neither will be necessary, as Mrs May would be able to negotiate a satisfactory deal before the time comes when they would have to be implemented.

The foreign secretary’s own preference is for the “maximum facilitation” scheme – known as Max Fac – which would use trusted trader arrangements and technology like number plate recognition cameras to avoid the need for border checks.

Environment secretary and fellow leading Brexiteer Michael Gove has said the Brexit backstop position on the Northern Ireland border was there “just in case”.

He told a conference on unionism in London today: “The whole point about the backstop is that it’s intended not to be implemented, but it’s there just in case.”