Boris Johnson backs Jacob Rees-Mogg on the day he threatened revolt against Theresa May

The foreign secretary said Mr Rees-Mogg is a 'principled and dedicated' politician

Boris Johnson has given his public backing to Jacob Rees-Mogg on the day that the backbench Brexiteer threatened open revolt if the prime minister fails to deliver a clean break from the EU.

The foreign secretary praised Mr Rees-Mogg as “principled and dedicated” despite Rees-Mogg having written a newspaper article appearing to suggest Theresa May would be ditched if she goes against Tory backbenchers’ wishes.

Mr Johnson’s intervention on Twitter on Monday served to further highlight Tory splits on Europe, hours after two ministers in Mr Johnson’s own department publicly attacked Mr Rees-Mogg over his piece.

It also came amid irritation from some cabinet ministers at not being told that Downing Street had drawn up a new customs proposal before it was reported in the media.

The foreign secretary’s comments set the scene for a crucial week, as Ms May prepares for talks with EU leaders including Angela Merkel and a crunch cabinet at which she hopes to secure approval for her plans for future relations with the EU.

The prime minister was just finishing taking questions in the House of Commons on last week’s European Council summit when Mr Johnson tweeted his support to Mr Rees-Mogg.

He wrote: “It’s vital that all MPs are able to air their views on Brexit.

“Whatever your position, I hope we can all agree that @Jacob_Rees_Mogg is a principled and dedicated MP who wants the best for our country.”

Mr Rees-Mogg faced a backlash after using a column in the Daily Telegraph to liken the Ms May to 19th-century Tory leader Robert Peel – forced to resign after joining forces with opposition parties to defeat his own MPs.

Ministers on Mr Johnson’s Foreign Office team led the charge, with Sir Alan Duncan hitting out at the Brexiteer’s “insolence” in “lecturing and threatening” the prime minister as “just too much”.

Alistair Burt tweeted: “Enough. Just tired of this endless threat and counter-threat.

“Why don’t we want the best for the UK than for our own ideological cliques? And there are others in this negotiation as far as I’m aware?”

Moments before Mr Johnson’s tweet, North East Somerset MP Mr Rees-Mogg, who also chairs the Brexit-backing European Research Group of Tory MPs, had again been pressing the PM to guarantee her plans would not leave the UK subject to the EU’s common external tariff, and looked unimpressed when Ms May failed to give an explicit commitment.

The new customs blueprint due to be discussed by cabinet on Friday is said to be distinct from both current options on the table, which provoked bitter disputes among Ms May’s top team.

The prime minister’s preferred “customs partnership” – which would see the UK collect EU tariffs – outraged pro-Brexit ministers and cannot be implemented before 2023 anyway.

The rival technology-based “max fac” plan would require border checks in Ireland and was torpedoed by HMRC, which revealed it would cost businesses up to an astonishing £20bn a year.

A government source said a third plan for customs arrangements had been devised, but insisted no details would be announced before the Chequers awayday.

The source denied reports that European Commission officials have already been briefed on the proposals.

But some members of the Cabinet were said to be miffed that the new plans arrival had been announced to the media before it had been cleared with them, further highlighting the combustible atmosphere at the top of government.

The Independent understands the cabinet will see Ms May’s white paper and customs proposals ahead of the Chequers meeting, but it’s unclear how much time they will have to consider it all before being asked to make a decision.

While having to handle a tetchy cabinet at home, Ms May must also hold meetings with key EU leaders in Europe, as she is to hold talks with Holland’s Mark Rutte on Tuesday and Ms Merkel later in the week.