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 canceled entirely got a boost from across the English Channel in a legal opinion by Manuel Campos Sánchez-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 Mrs. May opened five days of debate before the critical vote on Dec. 11 that will decide the fate of her proposal.