What was the Court of Session case about?
Boris Johnson advised the Queen last month to prorogue - or suspend - the parliamentary sitting from September 9. Following a meeting of the Privy Council, she agreed. The Prime Minister said this was a normal procedure before a Queen's Speech - when a government puts forward legislation. Mr Johnson's critics accuse him of trying to restrict the parliamentary timetable. Although five weeks would be lost, much of it covered party conference recess.
What did the court decide?
Three judges in Scotland's highest civil court, the Court of Session, ruled unanimously that Mr Johnson's decision was unlawful because it was 'motivated by the improper purpose of stymying Parliament' and stopping MPs holding the Government to account. One judge said Mr Johnson had failed to show a valid reason for proroguing Parliament.
What does the ruling mean?
In terms of its immediate legal effect, very little. The power to recall Parliament rests with Mr Johnson alone and he will wait for a ruling of the UK's highest legal body, the Supreme Court, which examines the case on Tuesday. But the ruling was a political bombshell and the Opposition called on Mr Johnson to resign.
Boris Johnson faces being left stymied by the latest court ruling over his prorogation of Parliament
What does No 10 say?
Officials insist the prorogation was to allow for a new Queen's Speech and not to stop MPs passing legislation against a No Deal Brexit. A source reportedly said that campaigners 'chose the Scottish courts for a reason' - comments seen as impugning the independence of the judges. But a spokesman for Mr Johnson later insisted the PM 'absolutely respects the independence of the judiciary'.
What do other courts say?
The High Court in London last week threw out a case based on the same arguments and brought by campaigner Gina Miller and others, including former prime minister, Sir John Major. They ruled that the matter was 'non-justiciable' because it was essentially a political question. Judges said courts should be 'slow to intrude' into areas that were a matter for government.
What happens next?
If the Supreme Court finds against the Government, the backlash would be fierce. Mr Johnson would have been found to have acted unlawfully and come under huge pressure from his opponents to quit. There is no legal sanction however.