Q&A: what did UK Supreme Court say and what happens next?

British prime minister Boris Johnson is under pressure to resign after the UK's highest court said he broke the law in a historic ruling that has deepened Britain's political crisis over Brexit. Here is what we know.

So what has just happened?

The UK supreme court ruled that Johnson’s advice to the Queen to suspend - or prorogue - parliament for five weeks earlier this month was “unlawful.”

All 11 judges said in an unanimous verdict that Johnson was wrong to stop MPs carrying out their duties in the run-up to Brexit on October 31st.

How big a deal is this?

It is huge. The UK’s highest court has ruled that a British prime minister has acted unlawfully and answered some fundamental questions asked about the country’s unwritten constitution in the midst of a political crisis around Brexit that has divided UK parliamentarians.

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It has thrown Johnson’s Brexit plan into disarray and handed a British prime minister, in office just 62 days, a humiliating defeat on excoriating terms.

How could the court rule on this issue?

Lady Brenda Hale, the supreme court's president, answered the legal question of whether the judges could rule on this case by saying that the matter was "justiciable" - it could be reviewed by the courts - deciding an issue that had divided the two lower courts, England's high court and Scotland's appeal court. Lady Hale said the question arises "in circumstances which have never arisen before and are unlikely to arise again."

What did the court say about Johnson’s decision to prorogue?

This was “not a normal prorogation,” the court ruled, as it prevented parliament from “carrying out its constitutional role” for five out of the possible eight weeks between the end of the summer recess and the UK’s planned date to exit the EU on October 31st and that the “effect upon the fundamentals of our democracy was extreme.”

Johnson’s direction to the Queen to prorogue parliament “had the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions without reasonable justification” and that it was “unlawful, void and of no effect.”

What did the court say about what should happen next?

The 11 justices said that it was for parliament and the speakers of both houses to decide what to do next.

What will happen next?

Speaker of the House John Bercow said the ruling was "unambiguous and unqualified" and that he would resume the business of the House of Commons for a sitting at 11.30am on Wednesday.

How has Johnson responded?

The prime minister, at the United Nations in New York, said he "strongly disagreed" with the ruling but would respect it. His aides said that he would not be resigning but would return home early from the US.

How did opposition parties react?

Labour leader Jeremy Corbyn, Liberal Democrats leader Jo Swinson and Nicola Sturgeon, the Scottish National Party leader, have all called on Johnson to resign as leader.

Swinson said the court ruling has shown that Johnson was "not fit to be prime minister". Brexit Party leader Nigel Farage blamed Dominic Cummings, Johnson's special adviser, for the move to suspend parliament, calling it the "worst political decision ever" and said that he "must go". Corbyn brought forward his speech to his party's annual conference in Brighton by a day to Tuesday afternoon to be able to return to Westminster.

What happens next?

Johnson’s authority as prime minister has been seriously undermined in embarrassing fashion by a 11-0 ruling by the UK’s highest court. Along with calls for his resignation, he will face questions about whether he misled the queen and whether the court’s decision has irreparably damaged his administration.

This ruling will overshadow the Conservative Party conference that starts next weekend and complicates his Brexit strategy to secure a better deal from the EU through brinkmanship in the run-up to the UK’s next deadline to leave on October 31st.

Instead, he faces a constitutional crisis at home while trying to press European leaders abroad for a reworked withdrawal agreement ahead of a key summit of EU leaders on October 17th and 18th from a weakened position.

The UK prime minister, if he survives, must try to find a compromise in parliament to get a Brexit deal before October 31st or face the risk of being the shortest-serving leader in British history.

If he is forced to delay Brexit and call an election, he could use portray the ruling in a campaign as another example of the establishment trying to block the UK’s exit and stymie the will of the people.