Brexit Secretary David Davis delivers a speech on Britain's security relationship with the EU after Brexit on June 6, 2018 in London | Leon Neal/AFP via Getty Images David Davis goes off-script on Irish border plans Brexit secretary casts doubt over the UK’s latest customs proposal, due to be presented to Brussels this week.

LONDON — Theresa May faces a showdown with senior Cabinet ministers after Brexit Secretary David Davis threw the government’s proposed Irish border “backstop" plan into doubt before it has even been officially unveiled.

In a hastily arranged speech in central London Wednesday, Davis urged the European Commission not to jeopardize the security of its citizens by refusing to adapt its rules to allow deeper intelligence sharing with the U.K. after Brexit than with any other country outside the bloc. Davis warned that U.K. citizens would be less safe outside the EU than they currently are unless the Commission refuses to back down.

However, the speech was overshadowed by the sudden rise in political temperature in Westminster after Brexiteers reacted furiously to a proposed Northern Irish border “backstop,” which the U.K. and EU have agreed to write into the withdrawal agreement.

Brexit negotiations stalled two months ago because the U.K. government has been unable to agree a position on their preferred post-Brexit customs relationship with the EU. With less than nine months to go until Britain drops out the bloc, pressure is building on the U.K. prime minister to make progress or risk leaving without a deal.

The so-called backstop clause, which was due to be presented to Brussels this week, was drawn up by Downing Street and circulated among Cabinet ministers Wednesday. As first reported by POLITICO, the proposal envisages the whole of the U.K. remaining subject to the EU’s common external tariffs and remaining aligned to single market rules for goods until a new customs relationship is in place.

Davis was furious that the clause — which is designed to guarantee an open border in Ireland in the event that a solution cannot be found on a future customs and trade deal — does not include a legally enforceable end date to ensure the U.K. is not trapped in a customs union in perpetuity.

Speaking at the Royal United Services Institute in Whitehall, Davis said the backstop plan has not yet been signed off by the prime minister’s “war Cabinet" — whose official title is the “strategy and negotiations” committee.

“It’s been through one Cabinet committee, it’s going through another one,” he said.

Davis added that while it would be “improper” to pre-empt discussions in the Cabinet committee, “I suspect it will be pretty decisive.”

Allies of Davis said the Brexit secretary felt “very significant and important conditions” had been attached to his support for the prime minister’s proposed backstop that had not been met.

Davis wants to make sure the U.K. has the power to “unilaterally decide when it considers the backstop has served its purpose,” one senior U.K. official said. “None of those conditions have been met in terms of the draft legal wording. It therefore doesn't have SN [strategy and negotiations committee] and Cabinet clearance. It does not have proper signoff.”

Brexiteers — including Davis, who will be in Brussels to meet with EU chief negotiator Michel Barnier next week, his department said — are concerned the EU will have no incentive to agree a future customs relationship if it has got what it wants in the backstop.

“Why are you ever going to bother about NCP or max fac because you've won the lottery,” the official said, referring to shorthand for two customs options being discussed by the government. “There's no way that will get through parliament. There's no way. It's a compromise too far.”

A senior MP and ally of Davis said the Brexit secretary is “very, very unhappy” about the lack of a clearly stated end date on the backstop proposal.

Brexit-supporting MPs are suspicious of any attempt by May’s chief EU adviser Olly Robbins to “bounce” the Cabinet into supporting a position that, while potentially unlocking the negotiations in the short-term, could bind the U.K. to EU rules and trade policy indefinitely.

“It’s come from [Olly] Robbins who thinks he can sell it to the EU to unlock the logjam,” the MP said. “But we’d never get out of it [the backstop arrangement]. We’d be left as a rule-taker, unable to trade with the outside world on free-trade terms.”

While there is serious concern about the backstop proposal among backbench Brexiteers, the MP and another Brexit-supporting colleague said they are not minded to speak out publicly against the prime minister at this stage, because of the imminent return of key Brexit legislation to the House of Commons.

Brexiteers’ first priority is the EU (Withdrawal) Bill votes in the House of Commons on Tuesday and Wednesday next week, where the government and Brexiteer MPs want to defeat 15 amendments handed down from the House of Lords, some of which could win support from pro-EU Tory rebels.

Brexiteers are wary of “rocking the boat” ahead of the vote, the second MP said, conscious that it might be seen as provocation by their pro-EU colleagues and increase the chances of them voting against the government on key amendments surrounding the No. 10’s customs strategy and the ability of parliament to shape the Brexit strategy.

At a meeting of the pro-Brexit European Research Group addressed by Foreign Secretary Boris Johnson on Tuesday evening, MPs expressed their support for the prime minister’s policy as set out in her Mansion House speech in March, the first MP said. The Mansion House speech did not set out the backstop proposal in its current form.