U.K. Prime Minister Theresa May is facing a major cabinet rebellion over Brexit, in a conflict that could destabilize the whole government, according to people familiar with the matter.

Brexit Secretary David Davis is said to be furious over May’s plan to tie the U.K. into European Union customs rules for an open-ended period of time after the country leaves the bloc next March. She argues it’s a necessary step to break the deadlock in talks with the EU.

The U.K.’s proposal was due to be given to the EU side Wednesday, but British negotiators are holding it back while May and Davis argue, officials said.

The issue rapidly became toxic, with Davis and his allies failing to quell speculation that he could quit over the row. Foreign Secretary Boris Johnson, Trade Secretary Liam Fox, and Michael Gove, the environment secretary, also object to the plan.

Davis’s Fury

Davis is the public face of May’s Brexit negotiating team and losing him at such a delicate moment in the talks would be a potentially disastrous blow for the prime minister’s strategy.

At the head of a fragile government with no automatic majority in Parliament, May also knows that the resignation of the minister responsible for her central Brexit policy could trigger a campaign to oust her among disillusioned members of her Tory party.

“To contemplate these negotiations continuing without David Davis would be deeply upsetting and deeply dangerous for the country and David Davis needs to stay where he is,” former Brexit minister David Jones told BBC Radio 4’s Today show. Backing his former boss’s position, he said that an open-ended customs deal “would just not be acceptable to the mass of the Conservative Party.”

Davis and his fellow euroskeptic ministers are ready to confront May over the policy at a crunch meeting of her Brexit “war cabinet” Thursday, the people said, speaking on condition of anonymity because the matter is sensitive. Davis won’t back down ahead of the meeting, although he and May could hold talks alone in a bid to hammer out their differences beforehand, the people said.

Davis did little to damp talk in the media that he could resign over the disagreement, when he said Wednesday that it is up to May whether he stays in her cabinet or not. Some government officials privately believe he will quit if May doesn’t back down, while others say they don’t know what he would do.

Latest Crisis

The latest crisis facing the premier is one she thought she’d successfully avoided. Last month, May persuaded Davis and the other pro-Brexit ministers in her inner cabinet that they should support her plan for a temporary customs arrangement with the EU, until a new system comes into force.

The plan was to have a fall-back option of keeping the U.K. aligned with EU tariff rules for longer. It was to be a last resort measure designed to solve the intractable problem of the Irish border. May’s officials drafted a paper setting down the proposal in writing, ready to be sent to the EU negotiating team for consideration this week.

But Davis apparently objected when he saw the document’s wording because although it stipulated that the extension of EU customs rules would be time limited, it did not say exactly when the arrangement would come to an end.

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Brexit backers in May’s party fear the result will be that the U.K. is forced to remain inside the EU’s tariff rules forever, destroying their vision for the country’s future outside the bloc. If the U.K. applies the EU’s tariffs on goods imports, it won’t be able to negotiate effective trade deals with other countries, euroskeptic Tories believe.

“It’s imperative that we don’t leave an open ended commitment,” said member of Parliament and former Tory leader Iain Duncan Smith. “The government’s chief negotiator is David Davis, and he wants to have an end date. We should support him and ensure there is an early end date.”

Time Limits

Brexit talks are deadlocked over how to avoid customs checks on the border with Ireland. The return of a hard border between the Irish Republic and Northern Ireland is something both the U.K. and the EU say they’re determined to avoid, amid concern checkpoints would endanger peace on the island.

May says she wants to fix the issue by negotiating a sweeping free trade deal with the EU that removes the need for border checks on goods. The EU is insisting that there must be a “backstop” plan in case these new trade terms don’t deal with the issue.

The difficulty for May is that while her pro-Brexit colleagues in London are desperate for a strict and clear end date to be agreed for the backstop plan, the EU side say any backstop must not be time-limited and should apply unless and until any suitable alternative can be found.