Boris Johnson, U.K. prime minister, speaks during the United Nations Climate Action Summit in New York, U.S., on Monday, Sept. 23, 2019.

The U.K. Prime Minister Boris Johnson has suffered a potentially fatal blow to his authority after the U.K. Supreme Court ruled that his move to shut down parliament was illegal, immediately triggering calls for him to step down.

Speaking at the Supreme Court in Westminster on Tuesday, the President of the U.K.'s highest court, Brenda Hale, said that the judges had found unanimously that the suspension was a matter for judges to decide upon and courts had the right to decide on the extent and limits of shutting down Parliament.

Hale then read out a ruling that found Prime Minister Boris Johnson had illegally suspended Parliament and that the Parliament had not, in effect been prorogued. Hale's statement went further, adding that the decision's effect on democracy was "extreme."

In an otherwise damning verdict, some relief was found for Johnson after Hale said the 11 judges had decided not to rule on Johnson's motive to prorogue.

Immediately following the decision, and speaking on the steps of the Supreme Court, the Scottish National Party MP Joanna Cherry said Johnson must now step down.

"His position is untenable and, for once, he should have the guts to do the decent thing and resign," she said.

Labour leader Jeremy Corbyn told an audience at his party conference in Brighton that Johnson should now consider his position and that U.K. MP's should open the doors of Parliament immediately.

"The PM has acted wrongly in shutting down Parliament. It demonstrates a contempt for democracy and an abuse of power by him... I will be in touch immediately to demand that parliament is recalled."

"I invite Boris Johnson, in the historic words, to consider his position and become the shortest-serving prime minister there has ever been."

The deputy leader of the main opposition Labour party Tom Watson echoed that sentiment on Twitter.

https://twitter.com/tom_watson/status/1176433403064999936

On Tuesday, in New York, Johnson told reporters he disagreed with the Supreme Court verdict but he would respect it. Striking a slightly defiant tone, Johnson said he would continue working toward Britain's exit from the European Union.

In a BBC interview, Johnson also hinted that he would prorogue the Parliament again in a bid to hold a queen's speech to set out a new legislative program.