LUXEMBOURG (Reuters) - Britain should be allowed to unilaterally revoke its departure notice from the European Union, an adviser to the bloc’s highest court said on Tuesday, before a landmark ruling in the coming weeks.

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The non-binding opinion for the European Court of Justice (ECJ) comes as the British parliament begins five days of debate on Prime Minister Theresa May’s proposed Brexit deal with the EU before voting on it next Tuesday.

“Advocate General (Manuel) Campos Sanchez-Bordona proposes that the Court of Justice should declare that Article 50 ... allows the unilateral revocation of the notification of the intention to withdraw from the EU,” the ECJ said.

“That possibility continues to exist until such time as the withdrawal agreement is formally concluded,” it said in a statement, meaning Britain would have to notify the EU that it has changed its mind before Brexit day, on March 29, 2019.

“The Advocate General emphasises that withdrawal from an international treaty, which is the reverse of treaty-making power, is by definition a unilateral act of a state party and a manifestation of its sovereignty,” the court also said.

“A member state which decides to withdraw is to notify the European Council of ‘its intention’ - and not of its decision - to withdraw, and such an intention may change,” it added. “It would be illogical to force that member state to withdraw from the EU in order to then have to negotiate its accession.”

The court, which tends to follow the advocate general’s opinion in its final decisions, did not give an exact date for the ruling but it is expected in the coming weeks.

OPPOSITION IN PARLIAMENT

May has agreed an exit pact and an outline of future EU-UK ties with the other 27 states staying on together after Brexit. But the proposed accord still needs the endorsement of the British parliament, where it faces stiff opposition.

The other EU states, as well as the bloc’s executive, want any revocation of Britain’s withdrawal notice to require the consent of the bloc, fearing that, otherwise, Britain or other would be leavers could use this tactic to win concessions.

Tuesday’s opinion rejects that argument but says that “good faith and sincere cooperation must also be observed” in any withdrawal of the exit notice.

The case was brought before the ECJ by Scottish politicians opposed to Brexit. They hope it could pave the way for a potential second Brexit referendum, giving voters the option to remain in the EU.

“That puts the decision about our future back into the hands of our own elected representatives,” said Jo Maugham, one of the lawyers involved in the case.

Both May and the EU have said that, if the British parliament votes this offer down, Britain would risk crashing out of the bloc with no deal in place to mitigate the damage. May says it would risk not delivering the Brexit that the British people voted for.

But the tentative deal is also opposed by advocates of a sharp break from the EU and she is struggling to get enough backing for it, even within her own party.

“Every effort is being made on both sides of the Channel to stop Brexit,” prominent ‘Leave’ campaigner Nigel Farage said after the ECJ statement.