Announcing the verdict on Tuesday, Lady Hale said the prorogation was “void and of no effect”, adding: “Parliament has not been prorogued.”
“It is for parliament, and particularly the Speaker and the Lord Speaker to decide what to do next,” she added.
“Unless there is some parliamentary rule of which we are unaware, they can take immediate steps to enable each House to meet as soon as possible.”
Lady Hale announced the court’s verdict 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 unlawful 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”.
“He’s misled Queen and country, and unlawfully silenced the people’s representatives,” she tweeted.
During a fringe event at the Labour party conference, where shadow foreign secretary Emily Thornberry was being interviewed, members broke into applause when the ruling was announced. One member shouted at the Labour frontbencher: “Get back to work.”
Speaking to reporters after the event, Thornberry said: “Parliament ought to be recalled and Boris Johnson ought to resign.”
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.
The pound rose against the US dollar following the ruling from the Supreme Court by 0.5% as markets reacted to the decision.