"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," U.K. Supreme Court president Brenda Hale announced.
House of Commons speaker John Bercow asked that Parliament's sessions be resumed "without delay."
"I welcome the Supreme Court ruling that the suspension of Parliament was illegal. The judges have rejected the Government's claim that closing Parliament for five weeks was simply normal practice," Bercow said and added that he will consult the Parliament leaders on the steps to follow "as urgently as possible."
After the announcement of the Supreme Court decision, Labor Party leader Jeremy Corbyn asked Johnson to "reconsider his position" at the head of the Government; however, the leftist politician did not say whether he would call for a vote of no confidence.
Nevertheless, the British Prime Minister will certainly face a difficult political scenario in the short term, for harsh criticism of his government is increasing, even within conservative ranks.
"No PM must ever treat the Monarch or Parliament in this way again," former Prime Minister John Major, a Tori who had joined anti-Brexit campaigners, said and added that Johnson should give an "unreserved apology" to the British Parliament.
Meanwhile, standing alongside the U.S. President Donald Trump at the fringes of the United Nations 2019 Climate Summit in New York, the current British PM said he will not resign.
"No, no, no," Johnson replied when asked by a U.S. journalist asked him about his political career.
While the British PM said that he respects the Supreme Court judges, he also stated that he disagreed profoundly with them. He said that as the law stood, Britain would still leave the European Union on Oct. 31.
"The exciting thing for us now is to get a good deal -- and that is what we are working on," Johnson commented.