The U.K.’s Supreme Court ruled Tuesday that Boris Johnson’s request for the Queen to suspend Parliament was “unlawful, void and of no effect,” in a severe blow for the British Prime Minister.

The ruling means lawmakers are free to reconvene, possibly as soon as Wednesday, allowing them to scrutinize Johnson’s Brexit preparations ahead of the U.K.’s Oct. 31 deadline to leave the European Union. They had previously been scheduled to return on Oct. 14.

Parliament, the Supreme Court said, “has a right to a voice.” Supreme Court President Brenda Hale continued: “No justification for taking action with such an extreme effect has been put before the court.”

“The Court is bound to conclude, therefore, that the decision to advise Her Majesty to prorogue [suspend] 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.”

The opposition Labour Party and the Scottish National Party both responded by calling for Johnson to resign. Parliament’s speaker, John Bercow, said lawmakers must “convene without delay,” adding he was making arrangements for lawmakers to return on Wednesday morning.

Before the ruling, the government said it would “abide by” whatever the court decided, and on Tuesday morning Johnson told the BBC he would respect the ruling. But Johnson has not ruled out suspending Parliament for a second time. The Supreme Court said it was for lawmakers — not the Prime Minister — to decide what to do next.

Johnson is currently in New York for the U.N. General Assembly. He was scheduled to return late Wednesday, but is reportedly planning to fly back earlier to address the crisis.

Johnson advised the Queen to suspend Parliament on Aug. 28, and Parliament was formally suspended for five weeks on Sept. 9. The suspension meant that lawmakers have since been unable to sit in Parliament or work on committees scrutinizing the government. Many observers said Johnson’s team took the decision to suspend Parliament in order to prevent lawmakers from blocking a “no deal” Brexit, which Johnson wanted to keep as a possibility in Brexit negotiations with the E.U. But days before the suspension officially began, lawmakers launched a concerted effort to take over the parliamentary schedule, and passed a law making a “no deal” Brexit illegal.

In the Supreme Court judgment on Tuesday, Justices ruled that the suspension of parliament was not a “purely political” matter, as the government had argued, and therefore could be ruled upon.

The judgment marks one of the most significant rulings in the U.K.’s constitutional history, marking a new precedent for the power the courts have to police squabbles between the government and parliamentarians.

Justices were considering competing judgments handed down by Scottish and English high courts. The English court ruled that Johnson’s advice to the Queen was a political matter, and not one for the courts. But the Scottish court ruled that Johnson’s advice was a matter for the courts — and that it was “motivated by the improper purpose of stymieing Parliamentary scrutiny of the government.”

What does this mean for Brexit?

Actually not a whole lot, partly because lawmakers acted fast to make “no deal” illegal before parliament was suspended — the outcome many observers believe Johnson was seeking to avoid.

Lawmakers were already scheduled to return to Parliament on Oct. 14, before the European council summit beginning on Oct. 17 which is being billed by officials on both sides as the time and place for compromise on a deal.

Lawmakers would have been able to scrutinize the outcome of that summit, including a possible new Brexit deal, even if parliament had remained suspended for the full five weeks.

But the Supreme court ruling will perhaps further damage Johnson’s already-weak position as Prime Minister. He currently heads up a minority government, having lost his majority in the run-up to the suspension of Parliament earlier in September. He made lots of enemies during that process: he expelled 21 members of his Conservative Party for voting in favor of the bill making “no deal” illegal; Johnson’s own brother Jo (also a lawmaker and government minister) announced he would leave the government and the party citing “unresolvable tension” between family loyalty and the national interest; and Amber Rudd, the work and pensions secretary, resigned from Johnson’s cabinet partly in response to the firings. Those expulsions and defections could reduce Johnson’s chances of getting a deal ratified by lawmakers, if he succeeds in returning with one from the European council summit in mid-October.

Will Boris Johnson resign?

It’s unlikely Johnson will resign in direct response to the Supreme Court decision, but on Tuesday morning he was facing calls from Conservative Party lawmakers to fire his divisive senior adviser, Dominic Cummings, the chief architect of the plan to suspend parliament and other controversial decisions like the expulsions of 21 Conservative lawmakers earlier this month.

The Prime Minister’s options, too, are narrowing. Thanks to the 2011 Fixed Term Parliaments act, which ended the ability of Prime Ministers to call elections at will, Johnson cannot call the election he wants without the support of opposition parties, which are denying it to him.

They won’t give him an election because it is now illegal for the U.K. to leave the E.U. without a deal, meaning Johnson must by law ask the E.U. for another Brexit extension if he can’t get lawmakers to ratify a Brexit deal before Oct. 31. Johnson has pledged never to request such an extension, and the Labour Party knows that the sight of the Prime Minister delaying Brexit would collapse his support, and therefore will not vote to hold an election before they see that happen.

To avoid doing that, one option Johnson has is to resign — but it probably won’t be in response to the Supreme Court’s ruling.

Write to Billy Perrigo at billy.perrigo@time.com.