LONDON — The British Supreme Court ruled on Tuesday that Prime Minister Boris Johnson illegally suspended Parliament, dealing him another heavy blow and thrusting the nation’s politics into even deeper turmoil, barely a month before Britain is scheduled to leave the European Union.
The unanimous decision, which upheld a ruling from Scotland’s highest civil court, said that the suspension of Parliament until Oct. 14 is void — means that the lawmakers are still in session and will continue the debate over Brexit that was short-circuited when Mr. Johnson asked the queen to suspend, or prorogue, Parliament for five weeks.
“The decision to advise Her Majesty to prorogue Parliament was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions without reasonable justification,” said Baroness Brenda Hale, the president of the court, speaking for the 11-judge panel that heard the case.
“The prime minister’s advice to Her Majesty was unlawful, void and of no effect,” she said. “Parliament has not been prorogued.”
The speaker of the House of Commons, John Bercow, who presides over debate, said the chamber will reconvene on Wednesday — 20 days earlier than the schedule Mr. Johnson had set.
Legal and political analysts had speculated that the court, which has historically avoided politics, might decide that it had no authority to rule on the prime minister’s actions, or might arrive at a mixed judgment.
Instead, it made a landmark decision to intervene in a fierce clash between Mr. Johnson and Parliament, and delivered a resounding defeat for the prime minister and an unequivocal victory to his critics.
The Supreme Court endorsed the judgment of the Scottish court that Mr. Johnson’s suspension was an effort to stymie debate before an Oct. 31 withdrawal deadline, which he had pledged to meet even if Britain and Brussels did not reach an agreement on the terms. It rejected the finding of an English court that the judiciary had no say in the matter.
The Supreme Court judges “have vindicated the right and duty of Parliament to meet at this crucial time to scrutinize the executive and hold ministers to account,” said John Bercow, speaker of the House of Commons, who presides over debate. “As the embodiment of our parliamentary democracy, the House of Commons must convene without delay. To this end, I will now consult the party leaders as a matter of urgency.”
Jeremy Corbyn, the leader of the opposition Labour Party, said the court had documented “a contempt for democracy and an abuse of power” by Mr. Johnson, adding that he would “demand that Parliament is recalled.”
Speaking at his party’s annual conference, Mr. Corbyn called on the prime minister to “consider his position” — an accepted, polite way in Britain of telling him to resign.
As a practical matter, it was not clear how much the decision would change the government’s immediate approach to Brexit. In the days before they were dispersed, members of the House of Commons pushed through a law — over the prime minister’s fierce opposition — that would prohibit Mr. Johnson from pursuing a “no-deal Brexit.”
But in symbolic terms, the court ruling was a stinging rebuke for the prime minister. It raised the question of whether he had misled Queen Elizabeth II in asking her to prorogue the Parliament. And it added to the perception that his Conservative government was running roughshod over Britain’s most hallowed political conventions in its zeal to extract the country from Europe.
Mr. Johnson has suffered repeated legal and political defeats since becoming prime minister in July. This month, after the Conservative Party lost its majority in the House of Commons, he called for early elections. But he failed to win the two-thirds majority he needed.
The Labour Party balked at supporting an election out of fear that Mr. Johnson would schedule it before Oct. 31, in the hopes of using a victory at the polls to push through a no-deal Brexit.
The Supreme Court decision, which came after three days of televised oral arguments, had been eagerly anticipated in Britain because of its political and legal ramifications.
“We’re in uncharted territory,” said James Grant, a senior lecturer in law at King’s College London.
Dr. Grant said he believed that it would have been dangerous for the Supreme Court to side with the English court. The suspension, he argued, clearly deprived the House of Commons of its responsibility to scrutinize the government’s policy on Brexit, an issue of critical national importance.
But other legal experts worry that upholding the Scottish ruling could set a troubling precedent, opening the door to a form of judicial review that is widely accepted in the United States, which has a codified Constitution and a Supreme Court that actively interprets it.
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, some legal analysts say, there is no going back.