The historic ruling – unanimously by all 11 judges – plunged his premiership into its deepest crisis so far as Britain faces a possible crash-out from the EU in just six weeks’ time.
The Prime Minister had sought to shutter Parliament until just two weeks before the planned EU exit date of October 31 and faced a barrage of criticism that he was trying stifle debate ahead of a no-deal Brexit.
He was accused of sparking a constitutional crisis by asking the Queen for the five-week prorogation, and had to fend off claims that he lied to Her Majesty about his true reasons for the move.
In a bombshell Supreme Court ruling, the eleven judges found that Mr Johnson’s actions were unlawful and had been done to block MPs from having a say, ordering that Parliament can be recalled immediately.
Delivering the damning ruling, Supreme Court president Lady Hale said: “This was not a normal prorogation of in the run-up to a Queen’s speech. 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 exit day on October 31.”
She said MPs and Lords had been blocked from quizzing ministers, taking evidence in committees and passing laws, and said the suspension of Parliament “took place in quite exceptional circumstances – the fundamental change which was due to take place in the Constitution of the United Kingdom on 31st October.
“Parliament, and in particular the House of Commons as the elected representatives of the people, has a right to a voice in how that change comes about. The effect upon the fundamentals of our democracy was extreme.”
Lady Hale said no justification for the exceptional step had been offered by the government, saying: “The court is bound to conclude, therefore, that 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.”
Lady Hale said the proroguing of Parliament on September 9 was “void”, and the speakers of the two Houses would decide what to do next rather than Mr Johnson.
MPs are now set to return to the Commons within days where ministers will be fiercely grilled over the prorogation, the landmark court case and the state of Brexit negotiations.
Mr Johnson has pledged “do or die” that Brexit will happen by October 31, with or without a deal.
However, MPs passed a bill earlier this month to block him from going ahead with a potentially catastrophic no-deal exit.
So Britain could be gripped by another historic legal battle within weeks if the Prime Minister seeks to get around the new law and the will of Parliament.
Speaker John Bercow has also made clear that he will fight to stop the Government disregarding Parliament.
Today’s ruling is a remarkable victory for a coalition of campaigners, including activist Gina Miller, SNP MP Joanna Cherry, former Prime Minister Sir John Major, and legal teams representing the governments in Scotland, Wales, and Northern Ireland.
They reached the Supreme Court after separate legal battles in the High Courts of England and Scotland.
Mr Johnson won at the High Court in London, when the Lord Chief Justice Lord Burnett and two other judges ruled that prorogation was a political decision and not a matter for the courts.
However, the Inner House of the Court of Session found the opposite, that Mr Johnson’s actions had been unlawful because it was “motivated by the improper purpose of stymieing Parliament” during the Brexit crisis.
The Government had denied this, claiming the Commons was being shut to pave the way for a Queen’s Speech to focus on domestic policies.
One of the most damaging revelations for Mr Johnson came after internal Downing Street memos on prorogation were made public.
In a handwritten note, he had dismissed Parliament’s September sitting as a “rigmarole” and claimed it was only introduced to show the public that MPs were “earning their crust”.
The Prime Minister was replying to a memo by one of his aides, Nikki Da Costa, who had publicly referred to prorogation in a magazine article as the “nuclear option” and suggested to the PM that Parliament could be suspended from September 9.
Lord Pannick QC, for Ms Miller, argued that an “improper purpose” was behind the controversial move, and attacked Mr Johnson for not provided a witness statement to the proceedings to support his case.
“It is a remarkable feature of these proceedings that the Prime Minister has not made a witness statement explaining why he decided to advise her Majesty to prorogue Parliament for a period as long as five weeks”, he said.
“The legal consequences of such a statement would have been almost inevitably an application to cross-examine – the legal consequences would also be that it would be a contempt of court of course for such a witness statement not to tell the truth.”
The Prime Minister came under attack from all sides during the three-day hearing, as lawyers from England, Scotland, Wales, and Northern Ireland accused him of abusing his powers and causing a constitutional crisis.
Sir John Major, who made an unprecedented intervention into the case, compared Mr Johnson to a dishonest estate agent and suggested the Prime Minister had not told the whole truth to the court.
“This is not a case where statements made can be taken to be the whole truth and nothing but the truth, and at their face value”, said the former PM’s lawyer, Lord Garnier QC.
Sir James Eadie QC, who fought the case for the Government, insisted that it would “not be constitutionally appropriate” for the courts to intervene over prorogation, arguing it would be a breach of the historic separation of powers.
“There are no judicial or manageable standards to test the lawfulness of a decision involving political or high policy”, he said.
Sir James and his legal team also raised the prospect that Mr Johnson could seek to prorogue Parliament again despite today’s defeat, if the court had left that option open with its ruling.
Closing the hearing last week, Lady Hale issued a reminder that the judges were not ruling on Brexit itself, and they were “solely concerned with the lawfulness of the Prime Minister’s decision to advise Her Majesty to prorogue Parliament on the dates in question.”
The Government insists that there has been progress in Brexit talks with Brussels.
EU leaders accept the “mood music” has changed for the better but are still dismayed at the lack of concrete new proposals from London for alternatives to the controversial Northern Ireland border “backstop” – the main obstacle in reaching a new Brexit agreement.
The stand-off is set for a showdown summit in mid-October in Brussels, with the Government expected to deliver new plans only shortly before then, with huge doubts remaining whether they will go anywhere near as far enough to satisfy the concerns of the EU and Ireland over no return to a hard border and protecting the single market.