The EU’s top court will say on Monday whether Britain can unilaterally halt Brexit, potentially offering a boost to those opposed to leaving the European Union on the very eve of a crucial and tumultuous vote in the British parliament.

In a brief statement on Thursday, the Court of Justice in Luxembourg said judges would deliver a ruling at 9 a.m. (0800 GMT) on Dec. 10 in a case brought by Scottish politicians who argue Britain can simply withdraw its plan to leave in March, without waiting for the approval of the other member states.

Prime Minister Theresa May is battling to get a Brexit deal that she negotiated with the European Union through parliament and insists there is no question of her stopping Brexit.

But in a vote due on Tuesday, the treaty faces opposition from lawmakers both for and against Britain leaving the bloc.

Acting with rare speed in a case only brought in October, a legal adviser to the court said on Tuesday that Britain could indeed make a U-turn entirely of its own accord. Such advice is usually, though not always, followed by the judges.

Clarification of Article 50 of the EU treaty matters because opponents of Brexit want a second referendum that gives the option of staying in the EU.

Stating that this does not need EU approval makes a new ballot more credible, according to supporters of a “people’s vote”. Britons voted in 2016 to leave the EU by 52 percent to 48.

EU leaders have long insisted they would welcome Britain changing its mind, but many EU officials and legal experts had believed that the approval of either all or most of the other 27 members states would be needed to halt Brexit altogether.

It is far from clear whether or how Britain could organize a new referendum, notably given the short time left until Brexit.

If May wins her vote on Tuesday, the withdrawal seems likely to proceed as agreed with Brussels last month. If she loses, her own position could be in jeopardy, there could be a move for a new election, or possibly to hold a new referendum.

Many warn, however, that it could stir unrest. Opinion polls suggest that any new majority for staying in the EU is narrow.