UK Prime Minister Boris Johnson cut his trip to the UN General Assembly in New York short in the wake of the ruling.

What happened on Tuesday?

Supreme Court judges unanimously agreed that the suspension of parliament was unlawful.

“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,” Supreme Court President Brenda Hale said, reading out the historic decision.

“Parliament has not been prorogued. This is the unanimous judgment of all 11 justices,” she added. “It is for parliament, and in particular the speaker and the (House of) Lords speaker, to decide what to do next.”

What did the speaker decide to do next?

John Bercow, the speaker of the House of Commons, appeared in Westminster shortly afterwards to announce MPs would sit on Wednesday.

They had previously not been due back to parliament until mid-October.

“I welcome the judgement this morning of the Supreme Court,” said Bercow. “That judgement is unanimous, that judgement is unambiguous and that judgement is unqualified.

“As you all now know that judgement, is that the prorogation of parliament was unlawful.

“Unlawful because it prevented or frustrated parliament in the discharge of its core duties. And it did so at a crucial time for our country. The citizens of the UK are entitled to expect that parliament does discharge its core functions, that it is in a position to scrutinise the executive, to hold ministers to account and to legislate if it chooses.”

How Prime Minister Boris Johnson react?

Johnson, in New York for the UN General Assembly, said: “And so I just want to say to everybody watching back home, for the avoidance of doubt I have the highest respect, of course, for our judiciary and for the independence of our courts.

“But I must say I strongly disagree with this judgement.

“And we in the UK will not be deterred from getting on and delivering on the will of the people to come out of the EU on October the 31st, because that is what we were mandated to do.”

Johnson spoke to Queen Elizabeth after the Supreme Court ruled he acted unlawfully when he advised her to suspend parliament just weeks before Brexit, a government official said on Tuesday.

Asked if it had been an apologetic call, the official declined to discuss the content of the conversation.

How did Johnson’s critics react?

They called on him to stand down.

“This unequivocal unanimous judgement from the supreme court makes clear that Boris Johnson is not fit to be prime minister,” said Jo Swinson, leader of the Liberal Democrat party. “He has acted unlawfully in trying to silence parliament, trying to silence the voices of the people because he does not want to be held to account, he does not want to have to answer questions about his disastrous Brexit policy, and in doing so he was prepared to mislead the Queen and indeed to mislead the whole country. He is not fit for office. He should resign.”

“Well, it’s extraordinary,” said Scottish First Minister Nicola Sturgeon. “Moments like this politicians have a tendency to engage in hyperbole but there is nothing by way of exaggeration in saying that this is the most significant constitutional judgement we’ve heard in our lifetimes.

“The supreme court this morning has found that the prime minister acted unlawfully, that he did so effectively to evade scrutiny and accountability, he had no good reason for doing so, and he did all of this at a time of political and constitutional crisis. It is genuinely exceptional and unprecedented.”

“It demonstrates contempt for democracy and an abuse of power by him,” said opposition leader Jeremy Corbyn, calling on Johnson to step down.

“Today is not a win for any individual or cause, it’s a win for parliamentary sovereignty, the separation of powers and the independence of our British courts,” said campaigner Gina Miller. “Crucially today’s ruling confirms that we are a nation governed by the rule of law. Laws that everyone even the prime minister (Boris Johnson) is not above. Do not let the government play down the seriousness of the judgement today, a unanimous judgement, they have spoken unequivocally. On what I say to the Prime Minister is to repeat Lady Hale’s (President of the Supreme Court) words, ‘the order was a blank piece of paper, parliament was not prorogued, MPs should turn up for work tomorrow and get on with scrutinising this government’.

“This is a marvellous day for democracy,” said Ian Blackford, leader of the Scottish National Party in the UK parliament. “This is a very clear indication to (prime minister) Boris Johnson and to the government that you cannot shut down parliament with impunity. This is an absolutely stunning judgement by the Supreme Court today. None of us anticipated that we would have results such as this. It is now very clear that we all have a job to do, we must be back in Parliament immediately. I know the speaker is going to be consulting with all the party leaders. We want to get back to work. And quite frankly on the back of this Boris Johnson must resign immediately.”

What happens next?

Bookmakers have slashed the odds of Boris Johnson leaving office before the end of this year. According to Oddschecker.com, after the decision the odds went from 6/4 to 10/11, implying a shift from a 22.2% to a 52% probability. Bookmakers currently have the outcome of Boris Johnson being the shortest-serving prime minister ever priced at 5/6.

The House of Commons said the parliamentary session will resume on Wednesday but added that there will be no PMQs as at least “two days notice of questions is required.”