Luxembourg - Britain could unilaterally decide not to leave the European Union, the bloc's top court ruled on Monday, one day before British lawmakers are due to vote on the Brexit deal London has struck with Brussels.

British politicians campaigning for a second referendum on Brexit hailed the ruling as a boost for their cause, while some commentators said it could help Conservative Prime Minister Theresa May in her bid to scare rebel lawmakers away from voting against her deal.

The decision to reverse Brexit would be a "sovereign" choice, the European Court of Justice (ECJ) argued. Subjecting it to the approval of other member states could end up forcing Britain to leave "against its will," the court said in a statement.

Disapprobation

British Foreign Secretary Jeremy Hunt dismissed the ruling, but co-litigant Alyn Smith, a Scottish National Party (SNP) member of the European Parliament, hailed it as a "huge win."

"I think it's irrelevant because just imagine how the 52 per cent of the country who voted for Brexit would feel if any British government were to delay leaving the EU on March 29," Hunt told reporters in Brussels.

"I think people would be shocked and very angry and it's certainly not the intention of the government," Hunt said.

Welcoming

Smith said taking the case to the ECJ had "worked better than [the SNP] could have hoped for."

"A bright light has switched on above an exit sign," he said.

Joanna Cherry, an SNP member of the British parliament, said she was pleased that the ECJ had "provided this lifeline to the UK parliament at this moment of crisis."

Liberal Democrat Brexit spokesman Tom Brake, who helped bring the case, said the ruling enabled the government to prevent a "chaotic no-deal" Brexit and helped bring the possibility of a second referendum "closer than ever."

The petitioners, who initially brought the case before the Scottish courts, had asked whether Britain can reverse its decision to trigger the Brexit countdown, arguing that lawmakers should be aware of all options when they cast their Brexit votes.

Countdown

Britain is due to leave the EU on March 29, 2019, two years after May invoked Article 50 of the EU treaty, triggering the countdown.

May's government had sought to have the case dismissed, arguing that the question is purely hypothetical since it has no intention of reversing Brexit.

But the Luxembourg-based judges rejected that argument.

The ruling is broadly in line with the recommendations of Advocate General Manuel Campos Sanchez-Bordona, a top advisor to the court.

It could, however, open the way for disgruntled member states to trigger Article 50 in future, in the hope of negotiating better membership terms.

The British parliament is due to vote on Tuesday on the withdrawal agreement and a political declaration outlining joint ambitions for the future relationship between Britain and the EU.

May has faced an uphill struggle to sell the deal to parliament. Hardline eurosceptics fear that it binds Britain too closely to the EU, while those in favour of a softer Brexit say it goes too far.