A left-wing critic of the European Union, he has played coy throughout the preparations for the withdrawal, known as Brexit.

Throughout the Brexit debate, Mr. Corbyn has ducked and weaved, trying to bridge the gap between the Remainers and Leavers in his own party while letting Mrs. May take the blame for the shambling effort to steer Britain’s departure.

His trick has been to oppose what Labour calls Mrs. May’s “Tory Brexit,” insisting that he could negotiate something better, while keeping alive the possibility of holding a second referendum, even if he has shown little enthusiasm for the idea.

Ahead of the 2016 referendum, he did the bare minimum for the pro-European campaign, and many believe he would be happy were Britain to leave the bloc — providing he is not seen as responsible for the exit.

“His ideal scenario is that May gets her deal through, he votes against it and is not tainted,” said Steven Fielding, a professor of political history at the University of Nottingham.

But after three large parliamentary defeats of the departure deal that Mrs. May negotiated with the bloc, that looks unlikely to happen.