A panel of 3 judges in the Court of Session ruled in favor of a cross-party set of politicians that contested the prime minister's movement, saying that the authority's information mentioned while looking for the prorogation arrangement is unlawful.
"Huge thanks to all our supporters & our fantastic legal team who have achieved the historic ruling that #prorogation is #unlawful #Cherrycase #Brexit".
Here's a look at some of the key questions following the ruling. This judgment will also be challenged in the UK's Supreme Court.
It is also likely to hear arguments arising from decisions in similar cases brought at the High Court in London under English law and the Northern Ireland High Court.
On the other hand, the judgment won't cause the immediate conclusion of parliament, despite demands from Labour, Scottish Nationalist Party, and many others, since it didn't dictate the cancellation of their prorogation.
"However, when the manoeuvre is quite so blatantly designed "to frustrate parliament" at such a critical juncture in the history of the United Kingdom I consider that the court may legitimately find it to be unlawful".
Responding to the latest court ruling on Wednesday, Labour's Shadow Brexit Secretary Keir Starmer told the BBC that Johnson should reopen the Houses of Parliament "this afternoon or tomorrow", so that MPs can debate what should happen next on Brexit.
Downing Street made clear that he had no intention of reversing the Commons prorogation at this stage - but would recall Parliament if the Supreme Court ruling went against the government.
Lord Carloway, Scotland's most senior judge, has said the advice given by the British government to Queen Elizabeth II concerning the prorogation of parliament was "unlawful".
"The mechanics of that depend on what the court says".
The court might say that UK Parliament was never prorogued at all in the eyes of the law and so is actually still sitting after all.
On Wednesday evening the MPs and peers involved in the Scottish court action attempted to intensify pressure on the government to reconvene parliament.
All three First Division judges have decided that the PM's advice to the HM the Queen is justiciable, that it was motivated by the improper goal of stymying Parliament and that it, and what has followed from it, is unlawful...
Yet before the prorogation MPs successfully passed legislation demanding Mr Johnson ask the European Union for a Brexit extension if a new deal can not be reached in time.
"So, if it were to be the case that the Government had misled the Queen about the reasons for suspending Parliament and the motives for it, that would be a very serious matter indeed".
A legal challenge that argued the government's Brexit strategy would damage the Northern Ireland peace process was dismissed on Thursday.
However, prorogation has reduced the opportunities for MPs to scrutinise the Government - and has cut short the time remaining to debate Brexit in the Commons before October 31.