EDINBURGH (Reuters) - Scotland’s Court of Session said on Thursday it has asked the European Court of Justice (ECJ) for a preliminary ruling on whether the British parliament can change its mind about leaving the European Union without the bloc’s agreement.

FILE PHOTO: An anti-Brexit demonstrator waves flags outside the Houses of Parliament, in London, Britain, September 10, 2018. REUTERS/Hannah McKay

Britain’s Conservative government is struggling to reach a deal with the EU on how it will leave the world’s biggest trading bloc as the March 29 exit date approaches.

Petitioners, who represent Scottish voters, successfully argued to the Scottish court that if parliament is to vote on the government’s eventual Brexit deal, it needs legal certainty on whether the process can be reversed without permission from the other 27 EU states.

The Court of Session has asked the Luxembourg-based ECJ, which rules on the meaning of EU law, for a preliminary ruling, the court said in a statement.

The British government, which has said it has no plans to reverse Brexit and therefore does not need to know, said it was disappointed by the decision.

“We are giving it careful consideration,” a government spokeswoman said. “But as the government has repeatedly said, we are committed to implementing the result of the referendum and will not be revoking Article 50.”

The Article 50 withdrawal clause in the EU’s Lisbon Treaty can be reversed with the permission of the other 27 EU members.

But it does not specify whether the exit process can be unilaterally reversed, although former British diplomat Lord Kerr who drafted it has said Britain can change its mind at any stage before the final exit date in 2019.

Scottish lawmakers opposed to Brexit filed a petition to the Court of Session, Scotland’s highest court, last year.

They represent electoral areas in Scotland which voted strongly to remain in the EU in the June 2016 referendum. The United Kingdom as a whole voted to leave.

The Scottish National Party, the strongest political party north of the border, has opposed leaving the bloc.

Given the urgency of the Brexit deadline the ECJ hopes to make a decision on the case before Christmas, sources close to the case told Reuters.

The government could in theory have presented an appeal to this stage of the process, but have not done so.

“This ship sailed yesterday - when the reference was sent to Luxembourg - and the government wasn’t on it,” Jo Maugham, a lawyer who is funding the Scottish lawmakers, said on Twitter.

“We believe it’s now too late.”

The Scottish petitioners argue that legal certainty about the process is needed in advance of any British parliamentary vote on an EU withdrawal deal because no country has ever before withdrawn from the European Union.

Brexit supporters say that after the vote to leave, any attempt to halt the process would be anti-democratic. But opponents say the country should have a right to pass final judgment on any exit deal negotiated, and the Scottish challenge is one of the most robust legal challenges to it so far.