LONDON — Britain can legally cancel its decision to leave the European Union and remain under its current terms, the bloc’s top court ruled on Monday, throwing a lifeline to those who still hope to reverse the withdrawal, known as Brexit.
The decision, from the European Court of Justice, confirmed a recommendation last week by one of the court’s senior legal advisers. It comes one day before the British Parliament is scheduled to vote on a withdrawal plan negotiated by Prime Minister Theresa May.
Most analysts predicts that Mrs. May will lose that vote, with dozens of lawmakers from her own Conservative Party planning to oppose the deal — an outcome that could throw the Brexit process into flux and British politics into a deep crisis.
The government has felt compelled to deny speculation that the vote could be postponed because of the strength of opposition.
Mrs. May started the two-year exit process, under Article 50 of the European Union’s treaty, in March 2017, and Britain is scheduled to leave on March 29. She has vowed to carry out the will of voters, who narrowly approved withdrawal from the union in a 2016 referendum.
But the European Court ruled that Britain could withdraw its Article 50 submission before that date, as long as it did so in accordance with the country’s democratic and constitutional arrangements, and communicated the decision in writing. The reversal would not require the approval of the 27 other member countries, the court said.
The decision is a boost for those who still hope that Brexit can be stopped, particularly if Parliament votes against Mrs. May’s deal. It came a day after thousands of people gathered in London for competing protests by supporters and opponents of Brexit, demonstrating that the issue has lost none of its divisive potency.
Britain has received several special concessions as part of its current membership terms, including one that allows it to opt out permanently from any obligation to join the European Union’s single currency, the euro. It also gains a valuable rebate on some of its annual budget contributions.
Were it to remain in the bloc, these membership terms would be kept, the court ruled. The ruling was made under a special, expedited procedure because of the urgency of the situation.
The case was taken to the court by a group of politicians who want to keep a close relationship with the European Union, or to reverse Brexit, including Alyn Smith, a member of the European Parliament from the Scottish National Party. In a Twitter post, he described the judgment as a “total vindication”
Even so, a decision to stop Brexit remains some way off. It seems likely that remaining in the union would require lawmakers to decide to hold a second referendum on whether to reverse the 2016 decision. So far, there is no obvious majority in Parliament for that step.
On Monday, Mrs. May was still fighting to win the vote scheduled for Tuesday and trying to win further promises from the European Union in order to reassure some pro-Brexit British lawmakers who are worried that her deal could leave Britain beholden to some European Union rules indefinitely.
Michael Gove, the environment secretary, told the BBC on Monday that the vote would go ahead as planned, and that Britain had no intention of reversing its decision to leave the European Union.