THERESA May is fighting to prevent a civil war engulfing her government, after Boris Johnson backed a leading Tory who warned she faced a rebellion over Brexit.

Amid a downward spiral in cabinet discipline, the Foreign Secretary praised Jacob Rees-Mogg as a “principled and dedicated MP who wants the best for our country”.

It followed Mr Rees-Mogg, chair of the European Research Group of Brexiter MPs, telling the Prime Minister to deliver on her own promises or risk a government collapse.

In a newspaper article denounced by other Tories as blackmail, Mr Rees-Mogg said the UK must leave the customs union, single market and ambit of the European Court of Justice.

Referring to Friday’s cabinet meeting at Chequers to finalise the UK blueprint for Brexit, he said “many MPs would vote against” any plan that did not deliver.

He highlighted the demise of Tory PM Robert Peel, who went against his own MPs in the 1840s over trade, never returning to power, and leaving his party out of office for 28 years.

Foreign Office minister Sir Alan Duncan condemned Mr Rees-Mogg's “insolence” and said “the ideological right... should pipe down”.

His fellow FCO minister Alan Burt said Mr Rees-Mogg was part of an “ideological clique” and said he was “tired of this endless threat and counter-threat”.

Tory MP Simon Hoare added: "The hectoring nonsense / blackmail has to stop. Tories are common sense pragmatists not dogmatic vestal virgins."

But on Monday afternoon, the Foreign Secretary pointedly tweeted his support for Mr Rees-Mogg.

He said: “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.”

The infighting worsened as it emerged Mrs May had been forced to drop two previous options for a customs deal with the EU, after her Remainer and Brexiter factions couldn’t agree.

She now plans to push a third option on Friday, likely to be a form of soft Brexit, after her chief Brexit negotiator, Olly Robbins, reportedly admitted the EU would not grant the bespoke Brexit option the UK government has been seeking for over a year.

EU officials are already said to have dismissed the latest idea as yet more "cake", a reference to Mr Johnson's plan for the UK to have its cake and eat it.

With only six weeks of negotiating time until the EU and UK are meant to agree the shape of Brexit, there are fears a deal may drift from October to December.

There were also reports some ministers were infuriated by the “magic” third way being sprung on them, accusing Mrs May her of treating them like “idiots”.

Nevertheless, the PM promised MPs there would be an agreement on Friday, with a detailed white paper next week setting out the “strong partnership” the UK wanted with Europe.

"The EU and its member states will want to consider our proposals seriously," she said.

“We both need to show flexibility."

Labour leader Jeremy Corbyn said Mrs May’s primary duty was to safeguard jobs and living standards “not to manage the latest division in her Cabinet”.

After meeting Mrs May in Downing Street, DUP deputy leader Nigel Dodds was asked about his party’s support for the government, he said: “We don't give blank cheques to anybody. We want to see a proper Brexit which fulfils the referendum result.”

SNP Westminster leader Ian Blackford: “It’s high time Theresa May reined in her chaotic Cabinet, and brought forward a credible plan that puts the interests of the UK ahead of her party’s hard Brexiteers.”

TUC general secretary Frances O'Grady likened the cabinet to “Dad's Army”.