The petitioners in the case - six Scottish MPs, MEPs and MSPs, along with Devereux Chambers' Jolyon Maugham QC - argue the case will help clarify the realistic options for MPs who will vote on Theresa May's "Chequers deal" a week today.
May is due to address Parliament Tuesday, opening five days of debate before a December 11 vote on the divorce agreement.
The deal, endorsed last month by the 27 other European Union leaders, lays out the terms of Britain's departure from the bloc on March 29 and sets the framework for future relations with the EU.
Defeat would leave the United Kingdom facing a chaotic "no-deal" Brexit and could topple the prime minister, her government, or both.
In an opinion statement to judges at the European Court of Justice, published on Tuesday, Advocate General Campos Sanchez-Bordona pointed to a "third way" that was now open to the United Kingdom, which is facing the real prospect of a no-deal Brexit amid massive parliamentary opposition to the withdrawal agreement reached between the British government and the EU-27 group of nations in November.
The ECJ has not yet made its ruling on the matter, but the breakthrough for pro-European Union activists in the United Kingdom came after the European advocate general gave his legal opinion on the matter.
The case comes as pressure builds from Brexit opponents for a second referendum on the decision to leave.
Campos Sanchez-Bordona, the European Court of Justice's advocate general, told the EU's top court it should allow the United Kingdom to withdraw its notice of intent to leave the bloc. Lawmakers are voting on a motion finding the government in contempt of Parliament for refusing to publish the full guidance from Attorney General Geoffrey Cox.
Alyn Smith MEP, one of those who brought the case, said the opinion issued by a European Court of Justice (ECJ) advocate general shows that "we now have a roadmap out of the Brexit shambles". "Over 100 Conservative MPs have said they are not going to back the deal, the Labour Party have said they are not going to back the deal". 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.
This would mean that the two-year period to achieve an agreement - which began in March 2017, when Article 50 was triggered, and is due to end in March next year - could be extended. The Bank of England warned last week that a no-deal Brexit could plunge Britain into a severe recession. The full courts ruling should be available much more quickly than normal because the court is dealing with the case on an expedited basis.
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.
The parliamentary showdown delayed for several hours the start of debate on the Brexit deal.
The European Union institutions also oppose the Scottish case, fearing member states will tempted to launch their own speculative exit bids to extract concessions from Brussels - only to reverse course.
Mr Maugham said in a statement today: "This puts the decision about our future back in the hands of our own elected representatives, where it belongs".