LONDON — Britain hurtled into unknown political territory on Tuesday when Parliament, for the second time, rejected Prime Minister Theresa May’s plan to quit the European Union, leaving her authority in tatters and the country seemingly rudderless just 17 days before its scheduled departure from the bloc.
Mrs. May had hoped that last-minute concessions from the European Union would swing the vote in her favor, but many lawmakers dismissed those changes as ineffectual or cosmetic and voted against the deal, 391 to 242.
After the vote, the prime minister defended her agreement as the “best outcome” for the United Kingdom and showed her frustration in addressing the lawmakers, who are scheduled to vote later this week on whether to seek an extension to leave the bloc.
“Let me be clear that voting against leaving without a deal and for an extension does not solve the problems we face,” Mrs. May said. “The E.U. will want to know what use we mean to make of such an extension, and the House will have to answer that question.”
Did Parliament want to remain in the European Union, she asked, or hold a second referendum? Or leave with some other deal?
“These are unenviable choices,” she said, “but thanks to the decision the House has made this evening, they are choices that must now be faced.”
The deal that Parliament rejected would have eventually given Britain power over immigration from Europe, but kept the country in the European Union’s customs and trade system until at least the end of 2020.
Tuesday’s vote, while expected, deepened an already profound crisis over the biggest peacetime decision to confront a British government in decades.
Mrs. May, who was forced to argue for her plan in a croaking voice because of a head cold, has essentially ceded control of events to Parliament, at least for now, with important votes coming on whether to bar a no-deal Brexit and whether to request the extension, something many analysts say is now inevitable.
In another admission of her weakness, Mrs. May said after the vote that she would allow Conservative lawmakers to vote as they saw fit, without a party directive, on a no-deal Brexit.
The defeat threatens Mrs. May’s hold on her office. Under party rules she cannot be challenged for the leadership by Tory lawmakers until December. But there is always the risk of a cabinet coup if she mishandles the next steps.
“The thing about this drama is that it is both chronic and acute,” said Tim Bale, professor of politics at Queen Mary University of London. “It has been going on for months, we have had so many near-death experiences, and I can’t remember a time when so much has happened for so long and the tension has been so heightened.”
Mrs. May now faces a number of possible options, none of them particularly palatable. She might still try one last time to force her deal through, perhaps at the very end of the month, but until then she will face pressure to change course.
Some lawmakers want to take nonbinding votes on various alternatives to Mrs. May’s Brexit plans, like those that would keep closer economic ties to the bloc, similar to those enjoyed by Norway.
There is discussion about a second referendum to confirm public support for a Brexit deal as against remaining in the European Union. The opposition Labour Party now says it would potentially support some form of plebiscite.
But Mrs. May has been implacably opposed to a second vote, saying it would not solve the problem.
More realistically, there is speculation about the possibility of a general election to change the composition of a logjammed Parliament. Though he was skeptical about that possibility, Mr. Bale said it could not be discounted completely, particularly since opinion polls show the Conservatives with a comfortable lead over Labour.
“If Theresa May really has not got anything left in the locker, and the E.U. doesn’t seem willing to give her a second chance, you could see why some members of Parliament — seeing the Conservatives have opened up a lead against Labour — might think: ‘O.K., this is a do-or-die moment.’”
The Conservatives had a healthy lead on Labour in 2017, which is why Mrs. May called the snap election that turned out so disastrously with the loss of her Parliamentary majority.
In theory, if there is no agreement by March 29, Britain will depart the European Union without any formal deal. But Britain is ill-prepared for a disorderly and potentially chaotic exit, and lawmakers are so alarmed at that prospect that they voted in January against such an outcome in a nonbinding motion. Parliament will get that chance again in a binding vote on Wednesday.
Yet, while a clear majority opposes a no-deal Brexit, a small number of hard-liners still believe they can extract more concessions from Brussels by threatening that very outcome, which could damage continental economies.
So Parliament remains stuck, largely as it has been for almost two years. The difference is that now there is very little time to chart a new path.
While Parliament is expected to support an extension in the Brexit negotiations, the question will be for how long and to what purpose. All European Union leaders would have to agree to extra time for Brexit and, when they meet on March 21, they will want to know the reasoning behind any request.
On Monday Jean-Claude Juncker, president of the European Commission, argued that, in any event, Brexit should occur before May 23, the day of elections to the European Parliament.
For legal reasons, any extension beyond this date might require Britain to take part in that contest, he suggested. That is something most British politicians do not want to contemplate.
Tuesday began with some mild optimism from Mrs. May’s allies after last-minute concessions with the European Union were agreed to Monday night in Strasbourg, France. Pro-Brexit lawmakers offered a cautious initial response and promised to study the details overnight.
But the mood darkened when the attorney general, Geoffrey Cox, said Tuesday morning the last-minute pledges from the bloc did not fundamentally alter the legal status of one aspect of the agreement that pro-Brexit lawmakers find particularly objectionable.
As Mr. Cox’s opinion reverberated among British politicians, Mrs. May received another damaging loss when legal analysts for the influential European Research Group, consisting of Conservative, pro-Brexit lawmakers, said they would “not recommend accepting the government’s motion today.”
Then 10 lawmakers from Northern Ireland’s Democratic Unionist Party, whose support allowed Mrs. May to form a government, also announced their opposition to her plan.
In explaining his decision, Mr. Cox said the concessions from the bloc did “reduce the risk” of Britain being trapped indefinitely in the so-called Irish backstop. That is an insurance policy against a hard border in Ireland that is a central issue for opponents of Mrs. May’s deal because they think it could trap Britain inside the European Union’s economic rule book.
But Mr. Cox concluded Britain would have “no internationally lawful means of exiting the protocol’s arrangements, save by agreement,” he said.