U.K. Prime Minister Theresa May “agreed” to a compromise on the Irish backstop on Sunday “contingent” on the approval of her Cabinet, two senior EU diplomats said, but her ministers rejected the proposal.

The package of measures was aimed at providing the U.K. with greater reassurance that the controversial Northern Ireland backstop would not be permanent, so improving the chances of the deal passing muster with Brexiteer Tory MPs. It involved reaffirming the power of an arbitration panel created by the existing deal to suspend the backstop if one side is acting in bad faith, the diplomats said.

"She agreed," one senior EU diplomat said, "providing to have the backing of the Cabinet, which she didn't get." The diplomat said the agreement was on "a legal document" designed to provide “guarantees for good faith on both sides.”

The Cabinet's refusal was apparently tied to objections by U.K. Attorney General Geoffrey Cox, the diplomats said. May has tasked Cox with making a legal interpretation aimed at easing fears among British MPs that the U.K. could become permanently trapped in the backstop arrangement.

EU officials and diplomats have reacted negatively to Cox's role in the talks in recent weeks. One official complained that the attorney general has approached the negotiations with the condescending swagger of an English barrister — his profession before becoming an MP.

The official said Cox had even managed to offend the EU's generally unflappable deputy chief negotiator, Sabine Weyand, by calling her "my dear" in what was perceived as a sexist, patronizing tone. A U.K. official said: ‘The attorney general refers to many people as 'my dear' — both men and women. He's a friendly fellow and it's a turn of phrase, nothing more.’

The tentative compromise accepted by May over the weekend was aimed at boosting the chances of the Brexit deal — composed of the 585-page Withdrawal Agreement and the Political Declaration — winning approval from MPs in a vote scheduled for Tuesday evening in the House of Commons.

German Chancellor Angela Merkel described it to reporters in Berlin Monday, according to Associated Press, as "an important offer [that] has again been made to Britain."

The plan that was signed off provisionally by the prime minister was essentially the same as that put forward on Friday afternoon in a series of tweets by EU Brexit negotiator Michel Barnier.

In those tweets, Barnier focused on provisions in the Withdrawal Agreement regarding an arbitration panel that, under certain circumstances, could allow the U.K. to suspend its obligations under the backstop provision. He also proposed a "joint interpretive statement" to provide further assurances to the U.K.

2/5 The arbitration panel can already, under Article 178 WA, give UK the right to a proportionate suspension of its obligations under the backstop, as a last resort, if EU breaches its best endeavours/good faith obligations to negotiate alternative solutions. — Michel Barnier (@MichelBarnier) March 8, 2019

EU officials have long believed that provisions already included in the existing deal, which was agreed in November, should be sufficient to allay concerns about the Northern Ireland backstop, which is intended to prevent the re-creation of a hard border in all circumstances.

Asked whether a fresh offer had been made over the weekend that was rejected by the U.K., May’s official spokesman said: “I wouldn’t characterize it like that. Talks were ongoing over the weekend but those talks continue now.”

There were already signs on Monday of a new potential breakthrough as May arrived in Strasbourg to meet with Commission President Jean-Claude Juncker and chief negotiator Michel Barnier.

Without some last-ditch deal — and a positive legal analysis by Cox — the Commons is virtually certain to reject the Brexit deal for a second time. Such a rejection would then set up subsequent votes, first on potentially rejecting a no-deal scenario and then on requesting an extension of the March 29 Brexit deadline.

Cox's refusal to accept the tentative agreement over the weekend only added to growing anger in Brussels over the U.K. attorney general's recent role in the talks, which diplomats and other officials described as unhelpful and even toxic.

A second senior EU diplomat who confirmed May's tentative acceptance of the compromise said that Cox had refused to take "yes" for an answer.

“This is what the U.K was asking for but then it said it was not good enough,” the senior diplomat said. "Mr. Cox has not been helpful."

The diplomat compared Cox to “a divorce lawyer who is appointed to walk away from the marriage without obligations” but who “is not interested in fair deal.”

Asked about Cox, a third EU diplomat said: “He is criminal lawyer with no experience in EU law ... I have second thoughts on him, probably he’s eyeing his own political career.”

A fourth diplomat said Cox "even argued that the backstop is against human rights." That line of reasoning was particularly irritating for EU officials because the backstop in its current formulation was agreed in November to meet the U.K.'s demands. London had rejected Brussels' Northern Ireland-only backstop.

"It’s like that because London wanted it the way it is," the diplomat said.

Charlie Cooper contributed reporting.

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