LONDON — Under a giant portrait of the children of an executed king of England in the great hall at Chequers, Theresa May will face the most dangerous moment of her premiership when she finally sets out her plan for Britain’s future relationship with the EU on Friday.

May needs her ministers to unite around her plan for Britain's future relationship with the bloc in order to break the Brexit deadlock in Brussels and survive at home. Despite the political jeopardy of forcing hard choices — MPs are calling the Cabinet get-together the "body bag summit" because those who don’t agree with the PM will have to fall on their sword — senior government officials say she cannot duck the moment any longer.

The complicated plan, which has been briefed to ministers in private meetings all week, involves a new, closely intertwined customs arrangement with Brussels and full-scale regulatory alignment with EU standards for Britain’s manufacturing sector, while leaving the U.K.’s giant service sector free to diverge from the European model.

The proposal is designed to protect Britain’s multinational businesses, with supply lines crisscrossing the EU, while also guaranteeing an open border in Ireland.

If the prime minister fails to win Cabinet support for her proposals — even at the cost of high-profile resignations — the prospect of “no deal” rears back into view, jeopardizing May’s grip on power and opening up the possibility of a fresh U.K. election and a genuine political crisis.

“It’s strategic madness" — Senior government official

"Continued division is now worse than any of the options causing it,” wrote William Hague in a Telegraph op-ed Tuesday.

Brexiteer Cabinet ministers and officials, however, believe that by moving now, May has abandoned the U.K. trench with a peace offer without any guarantee it won't be immediately shot down by Brussels.

“It’s strategic madness,” one senior government official said. “What is to stop Brussels turning round in October and saying ‘thanks, but no thanks’? Then what, single market and customs union or nothing, that’s what. I cannot see this flying with the Conservative Party.”

Calmer heads

If the test for Brussels was how badly the prime minister’s plan lands with her Brexiteers, then it passed with flying colors.

As soon as the plan started to seep out of No. 10 on Monday, it was met with fierce resistance from Brexiteer Cabinet ministers and officials. In a bid to stamp their authority, senior officials in Downing Street warned hard-liners opposed to the plan that they should resign if they did not back the policy, one senior official revealed.

By Thursday afternoon, as the nature of May’s proposal began to spread more widely around the Westminster grapevine, an air of revolt was beginning to take hold among Brexiteer backbenchers.

David Jones, a former Brexit minister, said it would “lock the U.K. into the customs union and single market in perpetuity.”

“This is not what people voted for in 2016. This is not Brexit,” he added.

Former Cabinet minister Owen Paterson said the reported strategy was a “complete breach” of the Conservative 2017 election manifesto, while Andrea Jenkyns, who resigned from government earlier this year to focus on Brexit from the backbenches, said she would await further detail but added that she would be prepared to vote against a Brexit deal on such terms.

May's inner circle hopes the temperature will have calmed by the time ministers arrive at the prime minister’s country residence in the rolling green countryside of the Chiltern hills, 40 miles northwest of the hustle and bustle of Westminster.

This, after all, was the intended purpose of the English Dacha, given to the nation after World War I as a place of peace for prime ministers to think.

On the stained-glass window of the Long Gallery in the mansion, there is an inscription: “This house of peace and ancient memories was given to England as a thank-offering for her deliverance in the Great War of 1914–1918 as a place of rest and recreation for her prime ministers forever.”

When May’s Cabinet ministers descend on the 16th century mansion, few will enter in anticipation of a peaceful next few hours.

The prime minister and her most trusted Brexit officials, led by chief EU sherpa Oliver Robbins, who leads the Cabinet Office’s Europe Unit, will present the plan to the 28-strong cabinet in the mansion’s central hall — a grand wood-panelled room, dominated by an open fire and giant canvas paintings. Other discussions will take place in in the Great Parlour, with its Cabinet-style table.

“It’s hard to exaggerate the degree to which the antics of several ministers, including Boris Johnson, have drained away goodwill towards the U.K." — Charles Grant

May’s plan is to all intents and purposes a minor adjustment to the so-called “new customs partnership” that Brexiteers had already rejected out of hand three months ago — and which was subsequently called “crazy” by Foreign Secretary Boris Johnson.

The plan, May will tell her ministers, will be for the U.K. to remain, in the long-term, in close alignment with EU rules governing the single market in goods, while agreeing to collect tariffs on behalf of the EU at its borders.

In the short-term, a second senior official said, the U.K. will enter a “bridge” phase when the standstill post-Brexit transition ends in December 2020, which would keep the U.K. within a de facto customs union with the EU until the new solution — now known as the “facilitated customs arrangement” or FCA — is ready to be implemented.

“It’s going to take some time to put the physical and technological infrastructure in place,” the official said. Some experts think it could take five or more years although the government is talking in terms of 12 months.

D-Day for Brexiteers

The upshot is that many of the potential gains from Brexit extolled by its advocates in government — the U.K being able to set its own tariffs and product standards, making it a nimble player on the global trading stage — will be, in the eyes of Brexiteers, sacrificed in order to keep the U.K. in the EU’s economic orbit and protect existing supply chains for businesses. Regulatory alignment could also open the door to a far greater influence in the U.K. for the rulings of the European Court of Justice than Brexiteers ever envisaged.

The big question facing the leading advocates of Brexit in May’s Cabinet — Johnson, Brexit Secretary David Davis, Environment Secretary Michael Gove and International Trade Secretary Liam Fox — is: Can they swallow it?

Early reactions suggest real anger. “It’s a total sellout, a complete capitulation,” said one senior government official briefed on the details of the prime minister’s proposals.

But Charles Grant, director of the Centre for European Reform think tank, argues that Cabinet resignations might actually strengthen May's hand in Brussels.

“It’s hard to exaggerate the degree to which the antics of several ministers, including Boris Johnson, have drained away goodwill towards the U.K,” he said. “If May was to lose some of her ministers … she would be perceived as someone who was finally cutting to the chase, finally taking tough decisions.”

May's advisers expect the summit to “finish when it finishes,” according to one aide. Meetings will take place over both lunch and dinner, and in-between.

There have been no preparations made for the ministers to be able to stay over. Indeed, Chequers only has 10 bedrooms, so it would be a cosy set-up for even the most harmonious of Cabinets.

While one government aide suggested a second day may be required if “the clock ticks past midnight” with no resolution, Conservative MPs have been informed that a briefing on decisions made at Chequers will take place at 11 a.m. on Saturday back at Downing Street.

It’s a lot harder to have a discussion with everybody making meaningful contributions when there’s 30 of you in the room" — Minister

The fact the meeting is full Cabinet and not, as in previous crunch points, the 12-strong so-called Brexit "war Cabinet," points to an attempt to dilute more trenchant viewpoints and keep substantive discussion to a minimum. The "war Cabinet" (officially the Strategy and Negotiations Sub-committee) is now split roughly down the middle between soft and hard Brexiteers, and has been sidelined as a result, one Cabinet minister said.

“I think the dynamic will be very different this time round,” the minister said, speaking on condition of anonymity. “It’s not just about the balance. We were a small group last time. It’s a lot harder to have a discussion with everybody making meaningful contributions when there’s 30 of you in the room.”

Nevertheless ministers will try their best to push their own agendas, down to the most minute detail. One minister said they need to know the plan for trade, customs, supply chains and fishing rights in such depth that they would be able to explain to interested parties “what it means for scampi.”

Merkel meeting

In the run-up to the meeting, Cabinet ministers have been called in to Downing Street for meetings with May’s Chief of Staff Gavin Barwell, her Director of Communications Robbie Gibb and, in the case of some senior ministers, May herself.

European allies have also been brought up to date. May met Dutch Prime Minister Mark Rutte at his official residence, the Catshuis in the Hague, on Tuesday and on the eve of the summit on Thursday traveled to Berlin to square off with Europe’s most powerful leader, Angela Merkel.

Irish Foreign Minister Simon Coveney, meanwhile, met with Chancellor Philip Hammond, Gove and Cabinet Office minister David Lidington on Wednesday.

As for her fractious MPs, around 50 of the most die-hard Brexiteers from the European Research Group were briefed by Chief Whip Julian Smith on Wednesday morning in a parliament committee room, where in a tense meeting MPs expressed concern at a report by ITV News’ Political Editor Robert Peston about the "third way," which characterized it as the “softest possible Brexit.”

They were told Peston was “off beam,” according to one MP present, but the strategy outlined to POLITICO by officials speaking on condition of anonymity is consistent with the ITV report. Smith reassured MPs that the government is “determined to deliver Brexit” and that an European Economic Area-style relationship, fully inside the EU’s single market, remained “a no-no” the MP said.

Smith was clear about what the government’s position isn’t, the MP said, but those present found it “hard to say” precisely what the government’s position is. May and her advisers also separately met MPs from the pro-EU wing of the party. All eyes will now be on the publication of a white paper, expected next Thursday, which should lay out the government's settled position on the way ahead.

Precisely what that white paper will say depends on what May can persuade her restive Cabinet Brexiteers to sign up to at Chequers — at a moment of maximum danger.