By Alexis Flynn

LONDON -- Legal experts said Thursday the government faces an uphill task overturning the High Court's unanimous decision that Parliament must have a chance to weigh in on U.K. plans to exit the European Union.

With no dissenting opinion in the three-judge ruling for the government to chew on, Attorney General Jeremy Wright and his legal team will have to find a fresh angle of attack if they hope to convince the Supreme Court to quash their judgment.

The country's Supreme Court of 12 justices will hear the government's appeal in early December. But instead of a single, codified bill of rights, the judges will take as their guide hundreds of years of legal history. Unlike the U.S., Britain's constitution isn't codified in a single document, and legal authorities rely instead on court rulings and expert writings stretching back to the Magna Carta of 1215.

While proponents say that has allowed the U.K. system to evolve more seamlessly over time, it can throw up a multitude of answers to questions. And while the country's decision to leave the EU was clearly voiced in June's referendum, legislation passed in 2015 authorizing the vote to take place was unclear on how exactly the process would take effect.

"No one seems to have thought this through at the time," said Steve Peers, a legal expert at the University of Essex.

During the High Court hearings in October, lawyers for both sides spent three days burrowing through arcane judgments to prove their points. These included a 1920s ruling about compensation payable to a hotel owner whose business was requisitioned during World War I; a 1976 case about the revocation of a commercial airliner's license; and an 1892 decision on how the rights of lobster fishermen in Newfoundland were affected by a British treaty with France.

King Henry VIII and the Glorious Revolution of 1688 also got passing mentions.

Ultimately, the court sided with the plaintiffs: Parliament is sovereign and is the ultimate authority when it comes to making and removing laws.

Making predictions about how the Supreme Court will judge the government's appeal based on their personal views is also difficult. Unlike their American counterparts, whose appointment is more closely scrutinized by lawmakers, the dozen judges who make up the court are appointed by a secretive selection committee made up of senior judges. Voicing any kind of opinion outside of court is strictly taboo.

Write to Alexis Flynn at alexis.flynn@wsj.com

(END) Dow Jones Newswires

November 03, 2016 18:48 ET (22:48 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.