LONDON — British Prime Minister Theresa May announced Monday that she would delay a vote on the withdrawal agreement she negotiated with the European Union, rather than face a devastating loss in Parliament that would have threatened both her Brexit deal and her political survival.

“If we went ahead and held the vote tomorrow, the deal would be rejected by a significant margin,” May conceded to a packed chamber in the House of Commons.

Nearly 100 members of her own Conservative Party had signaled they would vote against her half-in, half-out version of Brexit. Such a defeat would be hard for any prime minister to survive, but more so for May, who failed to win a majority for the Tories after a disastrous election campaign in 2017. 

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On Monday in Parliament, May instead chose the jaw-dropping humiliation of acknowledging the likely loss before it happened.

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In doing so, she extended her tenure long enough to give it another go with European negotiators. She insisted she had negotiated the best possible Brexit deal, but she agreed to return to Brussels this week and “do all that I can to secure the reassurances this House requires to get this deal over the line and deliver for the British people.”

By delaying the vote, May also prolonged the uncertainty over Brexit — whether, come March, there is her deal, no deal or no Brexit at all.

On news of Brexit chaos, the pound sterling plummeted and the stock markets in the United States and Europe dipped.

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May’s retreat did little to bolster her cause in Brussels, and it remains unclear what “reassurances” Europe can offer to placate her domestic critics.

E.U. leaders are exasperated by Britain’s endless domestic squabbling. Most view Brexit as a self-inflicted wound, worsened by lackluster leadership. And they wonder how long such a weakened prime minister can hang on to power.

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“We will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification,” European Council President Donald Tusk wrote on Twitter. “As time is running out, we will also discuss our preparedness for a no-deal scenario.”

Neither May nor the Europeans want a no-deal Brexit, although some hard-line Brexiteers say they are willing to suffer short-term pain for long-term ­independence. Economists have predicted that a no-deal “doomsday scenario” could result in food and medicine shortages; paralyzed trade and transport, including grounded aircraft; and a possible recession in Britain.

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In her remarks to members of Parliament on Monday, May said the “fundamental question” remains: “Does this House want to deliver Brexit?”

Some lawmakers shouted back: “No!”

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Several members of Parliament asked whether May would resign or call for a general election.

“No,” she responded.

While May emphasized the need to deliver on the result of the 2016 Brexit referendum and to protect British jobs, lawmakers were upset — angry, even — that the vote was pulled.

“The whole House wanted to debate this, we wanted to vote on it, the people expected us to vote on it. And the government have gone and run away and hidden in the toilets,” said Conservative Mark Francois.

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“The government has lost control of events and is in complete disarray,” said Jeremy Corbyn, leader of the opposition Labour Party. Despite knowing that the majority of British lawmakers didn’t support her deal, May “plowed on regardless,” Corbyn said. He added that “bringing back the same botched deal, ­either next week or in January . . . will not change its fundamental flaws.”

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Hard-line Brexiteers want a clean and complete break from the European Union, while Remainers say the proposed deal would incur economic costs to Britain without sufficient benefits.

If May somehow manages to get better terms from the Europeans, she could return to Parliament later this month or in early January, at the latest. 

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The House of Commons Twitter feed suggested that the ultimate deadline for a meaningful vote in Parliament is March 28, the day before Britain is set to leave the E.U. May suggested that Jan. 21 was the deadline that applied.

That assumes she lasts that long. On the sidelines, Tory leaders have been jostling for position to replace the prime minister if she resigns or is booted out in a no-confidence vote.

Labour leaders are hopeful for a chance to take power if a general election is called.

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Meanwhile, those who want Britain to remain in the E.U. have been looking for signs that Brexit might just go away.

On Monday, the E.U.’s highest court ruled that Britain could unilaterally reverse its decision to split from the 28-nation political bloc — a verdict that was expected but also gave a boost to anti-Brexit campaigners.

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There had been a legal question about whether a reversal would require the consent of the other 27 E.U. members, but the binding decision made clear that little would stand in London’s way should it decide it wanted to stay in the club. 

European diplomats say they are unwilling to consider any Brexit deal that leaves open the possibility of a hard border between Ireland and Northern Ireland. That means reopening the 585-page withdrawal agreement is probably off the table — even though that is the major source of the British objections.

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There is more flexibility to offer nonbinding assurances that the E.U. does not want to force Britain into Europe’s customs union after Brexit, a key Brexiteer worry. But since those declarations would have no legal value, they might not be enough.

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In Parliament on Monday, May struggled to make the case that preserving peace between north and south in Ireland was a promise that Britain must keep. Protestants and Catholics fought a 30-year sectarian civil war in Northern Ireland known as the Troubles.

Here are the “inescapable facts,” said May. “The fact that Northern Ireland shares a land border with another sovereign state. The fact that the hard-won peace that has been built in Northern Ireland over the last two decades has been built around a seamless border. And the fact that Brexit will create a wholly new situation: On 30 March the Northern Ireland-Ireland border will for the first time become the external frontier of the European Union’s single market and customs union.”

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The inflexibility on the European side is in part a result of a highly successful campaign by Dublin to paint any border with Northern Ireland as a red line. Irish leaders are unwilling to countenance such a change — and the other E.U. leaders are unwilling to side with a departing member of the bloc over Ireland.

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Diplomats involved in the negotiations acknowledge that both sides’ inflexibility may lead to a chaotic no-deal Brexit, with pain on either side of the English Channel — but they say giving in to British demands would also damage Europe’s economy.

May’s Brexit deal has been roundly criticized for tying Britain to E.U. rules and regulations for years to come. Hardcore Brexiteers say it would turn Britain into a “vassal state” and “a rule taker versus a rulemaker,” forever tied to Brussels but with little say. 

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May has stressed that her deal would allow Britain to control its immigration levels, which was a driving force in the Brexit vote in June 2016. But the compromise approach taken by May could limit Britain’s ability to make ambitious free-trade deals outside the continental bloc, as President Trump recently asserted.