LONDON — Prime Minister Theresa May on Tuesday broke with her own party, appealing to the opposition to work with her on a plan in a move that could keep Britain closely tied to the European Union after the country leaves the bloc.
Mrs. May made the announcement after her cabinet had met for seven hours, amid a growing sense of crisis. With only 10 days left until Britain is due to leave the European Union, she also said she would ask leaders of the bloc for an extension.
Parliament is deadlocked on the issue, and lawmakers are increasingly alarmed by the prospect of careering out of the bloc without an agreement.
Her overture to the Labour Party could mark a turning point in Brexit, as Britain’s exit from the European Union is known, ending months of stalemate between Mrs. May and Conservative hard-liners, who have adamantly refused to support the deal she negotiated with the European Union.
“This basically rips up the last two and a half years as far as her stance goes,” said Tim Bale, professor of politics at Queen Mary University of London.
“It seems the calculation has changed, and finally the country’s interests have been put above those of the Conservative Party,” he added.
Mrs. May’s deal with Europe has two parts: a withdrawal agreement, which details the practical terms of the divorce, and a political declaration, which outlines Britain’s future relationship with the bloc.
The prime minister said the withdrawal agreement must stand, even though Parliament has rejected it three times already. European leaders have insisted upon this.
Under the withdrawal agreement, Britain would remain in the European Union customs and trading system until at least the end of 2020.
Negotiations with the Labour Party will center on changes to the political declaration. However, these would be nonbinding and could be dismantled by a future British government.
Mrs. May’s plan had envisioned eventually severing ties with the bloc’s customs and trading system, and taking control over immigration from continental Europe.
Until now, she has refused to consider softening any of these so-called red lines.
The only option Mrs. May will not negotiate with the opposition is canceling Brexit altogether. Closer economic ties with the bloc through a customs union or membership in its single market are up for discussion and even a referendum confirming any deal has not been excluded, Mrs. May’s aides said.
Within an hour of Mrs. May’s announcement, pro-Brexit Conservatives began to express their dissent, amid reports that a number of cabinet ministers had argued against a further extension.
Whether the prime minister can win the cooperation of the Labour Party — and whether the European Union will grant her request for another delay — remained unclear.
“I’m very happy to meet the prime minister,” said Jeremy Corbyn, the Labour leader, after Mrs. May’s announcement.
“We recognize that she has made a move,” he added. “I recognize my responsibility to represent the people who supported Labour in the last election and the people who didn’t support Labour but nevertheless want certainty and security for their own future. And that’s the basis on which we will meet her and have those discussions.”
Still, to move forward with this plan, he will have to become involved in the tumultuous negotiation, and share responsibility for the success or failure of Brexit. Some Conservatives expressed dismay that the Labour Party would now be more involved.
“It is very disappointing that the cabinet has decided to entrust the final handling of Brexit to Jeremy Corbyn and the Labour Party,” said Boris Johnson, who resigned from Mrs. May’s cabinet and is considered a front-runner to replace her, in an interview with the BBC.
“I think the result will almost certainly be, if Corbyn gets his way, that we remain in the customs union so that we can’t control our trade policy, that there are huge areas of lawmaking we can’t control, and Brexit is becoming soft to the point of disintegration,” he said.
It has never been clear whether Mrs. May was willing to lead the country into a no-deal exit, a threat she raised repeatedly in tough-talking speeches at the start of negotiations.
A powerful faction in the Conservative Party, to which she has devoted most of her adult life, has shifted to a hard-line position, shrugging off warnings of severe economic consequences with a no-deal exit, and advocating what they called “a clean break.”
It was unclear whether Mrs. May had shifted as well. Her remarks on Tuesday, however, suggested that she had not.
“When it came down to it, she was just too sensible,” said Rosa Prince, the author of a biography of Mrs. May. “She’s got to be thinking of her legacy now, and if people are dying for lack of medicine, if there are food shortages, if Britain goes feral, that’s not how she wants to be remembered.”
Speaking at 10 Downing Street, Mrs. May said it was clear Britain would need an extension to Brexit, but “one that is as short as possible.”
Mrs. May’s immediate plan is to try to reach an agreement with Mr. Corbyn and get Parliament’s support for it before next week’s European Union summit in Brussels on April 10.
Alternatively, she said, Parliament would vote on an array of alternatives, with the government promising to accept the outcome.
Mrs. May hopes Britain might still be able to leave the bloc next month and avoid holding elections to the European Parliament. Failing that, she would ask for an extension that could be curtailed as soon as Britain had ratified its withdrawal agreement.
After Tuesday’s extraordinary cabinet meeting, ministers were held in Downing Street without access to cellphones until Mrs. May had spoken, a sign she wanted to get her version out before her fractious cabinet started leaking. Chilean wine was served as they waited.
Upon emerging, cabinet members told journalists that they had discussed the option of calling a general election, but that polling suggested that the Conservative Party might fare poorly if a new election was called now.
Mrs. May’s willingness to adjust her position on Brexit came “too late,” said Ayesha Hazarika, a political commentator and former Labour adviser. “This should have been done right at the beginning.”
She added, “It shows how catastrophically badly she has handled the chronology of the process.”
By this stage, Ms. Hazarika said, “the politics are too raw on both sides,” and Mr. Corbyn will be under intense pressure to call for a second referendum.
Mr. Corbyn himself is a longtime euroskeptic, but many of his party members are pushing strongly for a second referendum or for canceling Brexit altogether. Labour is also intent on holding a general election in the near future.
Mr. Corbyn, who has safely criticized the government from the margins, risks being held responsible for an unpopular result if he cooperates with Mrs. May, Ms. Hazarika said.
“Their strategy has been to have a clean-hands Brexit, the Tories will mess it all up,” she said. “If she’s co-opting him to be a co-pilot, does he want to take that responsibility?”
“He’s got a big strategic question to ask,” she added.
She said the next days were likely to bring all kinds of bizarre twists, like collaboration between Mr. Corbyn and Conservative backbenchers in an effort to bring Mrs. May down in a no-confidence vote.
“Anything is possible right now,” she said. “There are so many wheels within wheels right now. I have never seen anything like it.”