Not only did the court declare the prime minister’s action unlawful, it also declared the order itself, which Queen Elizabeth II issued at Mr. Johnson’s request, “unlawful, void, and of no effect.” The request, said the court’s president, Baroness Brenda Hale, might as well have been a “blank sheet of paper.”

Stephen Tierney, a professor of constitutional theory at Edinburgh University, said it was “astonishing” that the court had ruled decisively that it “can review something as fundamental as that, done by Her Majesty, as unlawful.”