"Unilateral revocation would also be a manifestation of the sovereignty of the departing member state, which chooses to reverse its initial decision".
Top Remainer lawyer Maugham - who wrote in triumph Tuesday morning that "Remaining in the European Union just got a whole lot easier" - moved to mobilise support against Brexit Tuesday morning, taking to Twitter to urge his 92,000 followers to "Please email your MP to let them know that the Advocate General has said we can cancel Brexit without cost".
A final verdict is expected within weeks.
Theresa May is opening five days of Brexit debate on her deal in Parliament today, before MPs get to vote on the agreement on Tuesday, December 11.
It can only take place in the two-year period from the member state's formal notification of triggering Article 50 to the EU Council, where its peers meet, he added.
Since Article 50 of the European Union treaty of Lisbon dealing with departing members is scant on details - largely because the idea of any country leaving the bloc was considered unlikely - a group of Scottish legislators wants to know whether the United Kingdom can pull out of the withdrawal procedure on its own.
Analysts said that if the European Court decides to support this opinion it will reinforce the hand of those who want the United Kingdom parliament to overturn the referendum result and stay in the EU.
He said parliament was not necessarily facing a choice between accepting Mrs May's deal or leaving the European Union with no deal, and that "there are other options, and we can stop the clock".
"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.
But her chances of winning majority backing for the deal look slim.
The case was brought forward in February by a group of Scottish politicians - Labour MEPs Catherine Stihler and David Martin, Joanna Cherry MP and Alyn Smith MEP of the SNP, and Green MSPs Andy Wightman and Ross Greer, together with lawyer Jolyon Maugham QC, director of the Good Law Project.
On Monday, the government published a summary of the legal advice, despite a parliamentary vote that had required the full advice to be released.
French government preparing to suspend fuel tax increases: French Prime Minister Édouard Philippe is preparing to suspend planned tax increases on fuel, according to a government source quoted by the AFP and Reuters news agencies. Mrs May's surrender document will not pass a House of Commons vote, and the obvious result of that would be for a "no deal" Brexit.
The opinion, which is not binding on the court, comes just a week after the case was heard at the ECJ following a referral from Scotland's highest civil court, the Court of Session.
Britain invoked the article on March 29, 2017 after voters backed Brexit in a national referendum the year before.
The government said that in light of the vote it would publish the advice from Attorney General Geoffrey Cox on Wednesday.
Bank of England Governor Mark Carney said Tuesday that British consumers could see their weekly supermarket bills up by 10 percent in a worst-case Brexit scenario that involves a 25 percent fall in the value of the pound.