Announcing the verdict on Tuesday, Lady Hale said the prorogation was “void and of no effect”, adding: “Parliament has not been prorogued.”
Lady Hale announced that the court’s judgment was the unanimous judgment of all 11 justices, adding the case is a “one-off”, having come about “in circumstances which have never arisen before and are unlikely to ever arise again”.
She told the court that “a decision to prorogue, or advise the monarch to prorogue, will be unawful if the prorogation has the effect of frustrating or preventing without reasonable justification the ability of parliament to carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive”.
Reacting to the verdict, Commons Speaker John Bercow said the house must “convene without delay” and that he would be consulting party leaders “as a matter of urgency”.
Liberal Democrat leader Jo Swinson said the decision by the Supreme Court confirms that Boris Johnson “isn’t fit to be Prime Minister”.
“He’s misled Queen and country, and unlawfully silenced the people’s representatives,” she tweeted.
Deputy Labour Leader, Tom Watson, called for Johnson to resign, a sentiment also expressed by Green Party MP Caroline Lucas.
Protesters outside the Supreme Court erupted into cheers following the ruling.
Chants of “reopen Parliament” and “Johnson out” quickly began as the news spread through crowds, who were watching livestreams on mobile phones outside.
Susan Rogers, 70, told PA: “I am really pleased, I assumed that they would come to this verdict.
“It really shocked me the liberties that were taken, with the lies and with the prorogation of Parliament.
“I think Parliament should reopen and start dealing with the problems in the country. Look at the homeless, look at the NHS.”
The prime minister ‘prorogued’ parliament for five weeks at the start of September, arguing the suspension would allow him to set out a new domestic agenda in a Queen’s speech.
But critics accused Johnson of trying to escape scrutiny in the run up to the October 31 Brexit deadline, with MPs not due to return to Westminster until October 14.
The Supreme Court judgement comes after challenges to the PM’s decision resulted in different rulings at courts in England and Scotland.
The Court of Session in Edinburgh ruled that Johnson’s suspension of parliament was “unlawful” following a case brought by a group of around 75 MPs and peers.
However, a challenge by businesswoman Gina Miller in the High Court in London was unsuccessful.