Britain, by contrast, relies on an unwritten set of traditions and conventions that have treated a sovereign Parliament as the supreme law of the land. Once the courts venture into the political sphere and begin passing judgment on Parliament’s actions, legal analysts say, there is no going back.

“If the court accepts the invitation to devise a legal rule, there will be no logical limit to the extent that the court might want to replace conventional rules with legal rules,” said Sir Jonathan Sumption, a former justice of the Supreme Court, who is a leading commentator on the court’s role in society.

“I question the need for a judicial intervention,” Sir Jonathan said, “but I think it may happen because people are shocked by what the government has done.” Mr. Johnson’s decision to suspend Parliament, he said, was the equivalent of “taking an ax to the political convention.”

Both sides of this case have unfolded in hours of legal arguments, which are being livestreamed by the BBC and other British news organizations (cameras in the courtroom are another difference between Britain’s Supreme Court and its American counterpart). Among those submitting arguments to the justices was a former prime minister, John Major, who has fiercely criticized Mr. Johnson’s actions.

The case has commanded the headlines in Britain. But the televised images of justices staring into computer screens — often interrupting the lawyers to figure out what page they needed to scroll down to — are worlds away from the spectacle in Parliament the previous week. It has served mainly to underscore the differences between the court and other British institutions.

For one, it was founded only in 2009, after a constitutional reform process initiated by Prime Minister Tony Blair, though its roots date back far longer, through its predecessor body, the Appellate Committee of the House of Lords.

Still, the court has none of the antiquarian traditions of Parliament or even other British courts. Barristers do not wear powdered wigs. The justices are seated behind a polished, modern semicircular table, not on a raised dais. Except for ceremonial occasions like the opening of Parliament or the swearing-in of a new justice, they do not wear robes, but regular business attire.