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.”