LONDON—Prime Minister Theresa May of Britain on Tuesday suffered a series of humiliating defeats in Parliament, the strongest sign yet that lawmakers are poised to reject her plan for exiting the European Union.

Also on Tuesday, those hoping that the withdrawal plan could be cancelled entirely got a boost from across the English Channel in a legal opinion by Manuel Campos Sanchez-Bordona, the advocate general of the European Court of Justice.

He said that under the European Union’s governing treaty, Britain could unilaterally cancel its withdrawal from the 28-nation bloc before the scheduled date of its departure, on March 29, and without having to obtain the consent of the other 27 member nations.

The setbacks in Britain’s Parliament came as May opened five days of debate before the critical vote on Dec. 11 that will decide the fate of her proposal.

In a significant move, lawmakers on Tuesday voted to give Parliament more control of the exit process should her plan be voted down. This could allow them to avoid the “no-deal” Brexit, a disorderly and economically damaging departure from the European Union without any agreement.

And for the first time in history, they voted that the government was in contempt of Parliament for failing to publish its full legal advice on Brexit — a text explaining the legal implications of the deal. That document will now be published, the government said.

In London, the growing assertiveness of lawmakers and the breakdown of discipline within the ruling Conservative Party does not bode well for May, who was already facing deep challenges in pushing through her plan, most analysts have concluded.

Defeat on Dec. 11 could force the prime minister to abandon her proposal or break up her government, or lead to a power struggle that imperils her leadership.

But May, who has won plaudits for her dogged persistence, is still fighting for what she believes is the best way to honour the outcome of a 2016 referendum decision to quit the bloc.

“We should not let the search for a perfect Brexit prevent a good Brexit that delivers for the British people,” May told lawmakers.

She insisted that the alternative to her plan was either a disorderly and economically damaging exit without any agreement, or no Brexit at all.

Jeremy Corbyn, leader of the opposition Labour party, argued that her plan would make Britons worse off and was a “huge and damaging failure.”

So divisive has Brexit proved that the two party leaders are unable even to agree on the terms of a TV debate on the issue; negotiations on a broadcast seemed to have collapsed on Tuesday as well.

Dozens of lawmakers in May’s Conservative Party are threatening to rebel, angered in particular about a so-called backstop plan to prevent physical checks of goods at the Irish border.

That would keep the United Kingdom in a customs union with the European Union, which critics say would mean that Britain would be neither fully in the European Union nor out, and that it would have no say in making rules that it would be bound by.

The prime minister faces opposition from across the political spectrum, as well as growing calls for a second referendum on Brexit, so that voters can have a final say on the terms of the country’s departure.

May’s one advantage is that her opponents are divided.

Some actively want the country to leave the bloc without a deal — or at least appear ready to risk that — while others prefer maintaining a close relationship with the European Union, much as Norway does.

A third group is pushing for the second referendum.

Pro-European Britons will be encouraged by the opinion of the advocate general in the European Court. His views are not binding, but are mostly followed by the full court in its final rulings.

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The opinion was written in connection with a case brought by a group of pro-European Scottish politicians.

If the advocate general’s opinion is confirmed by the full European court, Britain will have until March 29 to rethink its departure and remain a member of the European Union on the current terms, which include several concessions.

However, May has insisted that the country will leave as scheduled.

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