LUXEMBOURG (Reuters) - 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.

FILE PHOTO: An anti-Brexit demonstrator wears a combination of EU and Union flags outside the Houses of Parliament in London, Britain, November 26, 2018. REUTERS/Toby Melville/File Photo

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.

THIS DEAL, NO DEAL -- OR NO BREXIT

May said on Thursday that the alternatives to her deal, under which Britain will for now remain closely tied to EU regulations over which it has no say, were a disorderly “no deal” Brexit or no Brexit at all.

That echoed, notably, EU summit chair Donald Tusk last week. He has repeatedly said Britain can still stay.

Tusk will chair a regular summit next Thursday and Friday and, if May has lost the vote, EU diplomats expect leaders to offer her some prospect of further discussions on how to make the deal acceptable in London -- while making clear there is no time left for substantial changes to agreed texts.

But talk of a possible British U-turn has increased in Brussels in the light of the parliamentary opposition to May’s deal and of the ECJ opinion. Other governments are concerned that a deeply divided Britain would remain an awkward EU member. However, few appear willing to be seen to force Britain out.

As a result, officials and diplomats say, most are ready to postpone Brexit from March 29, at least by a few weeks, either to ensure an orderly withdrawal or, just possibly, give time for a second referendum. Few see the latter as likely, however.

The speed of the ECJ’s intervention, compared with rulings that typically take many months, prompted the likes of Brexit campaigner Nigel Farage to accuse it of meddling in politics.

ECJ President Koen Lenaerts has long predicted that Britain’s withdrawal will end up before his court. In taking on what is known as the Wightman case after one of the lawmakers who brought it, the court said on Oct. 19 that it aimed to rule before the British parliament takes its decision on the deal.