LONDON — A pervasive gloom has descended over even the most upbeat of U.K. officials involved in the Brexit negotiations.

“I’m not going to lie and say it’s great right now,” said one senior government aide, reflecting on the growing political turmoil facing the government.

In the space of 24 hours, Foreign Secretary Boris Johnson has openly challenged the prime minister over her preferred post-Brexit customs plan while the House of Lords has sent another wrecking ball through the government's negotiating aims.

Another high-ranking official was more blunt: “God help us all!”

At the heart of the growing pessimism in Whitehall: The political impasse over Britain’s preferred future customs relationship with the EU, which is threatening to tear down the government and the Brexit negotiations with it.

"Due to divisions within the government, these negotiations are in a shambles" — Jeremy Corbyn

Without Cabinet consensus on what the U.K. is trying to achieve, the negotiations in Brussels cannot progress.

The customs impasse is adding to concern in Whitehall that the U.K. will simply be left with the invidious choice of “no deal” or an ongoing customs union with the EU, binding the U.K. into trade policies over which it has no say — neither scenario of which the prime minister is likely to survive.

At weekly Prime Minister's Questions in the House of Commons, Labour leader Jeremy Corbyn taunted Theresa May over her foreign secretary's assessment of her customs plan as "crazy."

"Due to divisions within the government, these negotiations are in a shambles," said Corbyn.

Backbench Tory Brexiteer Jacob Rees-Mogg is careful not to openly make direct leadership threats but his caucus of MPs, the European Research Group, have made clear how dimly they regard May's preferred customs solution. For them it is as bad as remaining within the EU's customs union and so effectively goes against the Tory Party's manifesto commitments.

“Any MP who stood on the Conservative manifesto and did not say 'I don’t agree with our policy on leaving the customs union and single market' has an obligation to consider the manifesto very carefully and the commitments they may have made to their constituents very carefully," Rees-Mogg told the Telegraph. “They need to think very carefully about whether they might have misled their voters."

The fear among the most senior officials involved in the Brexit negotiations is that the political impasse in Westminster will mean that Britain is left humiliated, either forced to accept whatever Brussels leaves on the table as the clock ticks down to exit day on March 29, 2019, or — even worse according to some in Whitehall — being bounced into a customs union by the House of Commons.

“The worst of all worlds is being ordered to enter a customs union. We would have absolutely no leverage. Brussels would know we would ultimately have to take whatever they said, because the Commons had ordered us to do so, so we’d get no meaningful opt outs or anything like that. It would be a disaster,” one U.K. official said.

A second senior official involved in the Brexit negotiations said: “The truth is staying in the customs union is a terrible Brexit. It’s obvious — there’s no way of dressing it up. It is clearly worse than staying in the EU. It’s just a terrible Brexit. But if we don’t get a plan, the EU will lose patience and make the decision for us, leaving us no choice but to sign up or crash out.”

In a bid to find consensus among her most senior ministers who sit on the inner Cabinet sub-committee which sets the government’s policy on Brexit — the so-called “war Cabinet” — Theresa May has ordered civil servants to work up new versions of the two customs models on the table, the customs partnership favored by her and the Remainers, and the so-called "maximum facilitation" model backed by Brexiteers.

The former involves the U.K. collecting customs duties on the EU's behalf but refunding some traders whose goods are destined for the British market. The latter brings together a variety of technological solutions to obviate the need for customs checks at the border.

The new versions of both models — “Max Fac 2.0” and “Partnership 2.0” in Whitehall lingo — will seek to address the concerns of those on both sides of the Cabinet divide.

The Remainers, led by Chancellor Philip Hammond, are worried about the impact on the British economy of a straightforward departure from the customs union, particularly on the supply chains of British manufacturers. Business Secretary Greg Clark warned on Sunday that thousands of jobs are at risk.

The Brexiteers, in contrast, led by Johnson, Brexit Secretary David Davis and and International Trade Secretary Liam Fox, are concerned about the complexity of the customs partnership. They doubt that it can ever be made to work and fear it would limit the scope for future trade deals, thereby undermining one of the key tenets of Brexit.

“Lots of slightly inexpert people are groping around for ideas" — Senior U.K. official

“It’s totally untried and would make it very, very difficult to do free-trade deals,” Johnson told the Daily Mail in an interview published late Monday. “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 U.K. frontier.”

There is little scope for a “third way” model because the nature of customs is binary: you are either in a customs area or you are not, according to officials close to the negotiations. If the U.K. leaves the EU’s customs union there must be a customs border in Ireland or the Irish Sea. The choice is unavoidable, but both sides are currently sticking to their guns.

“Lots of slightly inexpert people are groping around for ideas,” said one senior U.K. official. “[Some] are practically doable, but surely politically suicidal.”