Lady Hale, the President of the Supreme Court, stunned Westminster as she said 11 judges had agreed that the suspension of Parliament was ‘void and of no effect’.
Commons Speaker John Bercow responded as he said Parliament will go back to work tomorrow morning while Mr Johnson has railed against the ruling and claimed the judges got their decision wrong.
What happens next? Here are the answers to all of the key questions:
Will Boris Johnson resign?
The Supreme Court’s ruling is highly embarrassing for Mr Johnson and puts the PM in completely uncharted territory.
The fact that he was found to have acted unlawfully represents a hammer blow to his premiership and has unsurprisingly prompted calls for him to quit.
Jeremy Corbyn, the Labour leader, pounced immediately after the ruling was read out as he said the PM must now ‘consider his position’.
Meanwhile, Nicola Sturgeon, the SNP leader and Scottish First Minister, said a premier with ‘any honour would tender his resignation’.
She said that if Mr Johnson will not do the ‘decent and honourable thing’ then MPs should try to force him out.
But Mr Johnson responded to the ruling by insisting he was right and the judges had got their decision wrong.
He had previously said that he had no intention of resigning if the court ruled against him and based on his hardline comments this afternoon that position has not changed.
The ruling represents a major set back for Mr Johnson, pictured in New York today, who is now facing calls to resign. He said he believes the court made the wrong decision
Will the PM now face a vote of no confidence?
The possibility of a vote imminently is receding, despite many MPs clamouring for Mr Corbyn to table one.
The responsibility for seeking a vote of no confidence rests with Mr Corbyn as the leader of the opposition, and he has said it will not happen until after the ‘threat of No Deal is taken off the table’.
Opposition leaders rejected Mr Johnson’s demands for an early election earlier this month because they did not want to go to the country before a No Deal Brexit has been ruled out.
But an anti-No Deal law is now on the statute book while rules relating to the holding of general elections dictate that there must be a 25 day campaign period.
That means any election caused by toppling Mr Johnson would not take place until after October 31 – and after the PM has been required by law to ask the EU for a Brexit delay should no agreement have been struck with the bloc.
It is important to remember that the UK must always have a prime minister: Even if Mr Johnson lost a vote of no confidence and resigned he would be expected to stay in post until a replacement has been chosen or elected.
Could a vote of no confidence succeed?
The vote would almost certainly be tight. Mr Johnson would expect to count on the support of the overwhelming majority of Tory MPs although today’s Supreme Court ruling could make some think long and hard about backing the PM.
Mr Johnson would also likely be supported by a number of Labour Brexit-backing MPs as well as the DUP.
On the other side, if Mr Corbyn was to launch a push to get rid of Mr Johnson he would likely only do so if he believed all the other opposition parties were on board.
Lib Dem sources have suggested they could now back a vote of no confidence while the SNP would leap at any opportunity to boot out Mr Johnson.
The parliamentary arithmetic means that the result could ultimately come down to how a group of 21 Tory rebels who were stripped of the whip by the PM after backing the anti-No Deal law would vote.
If they decided to vote with the opposition Mr Johnson could be in big trouble.
Tory backbencher Tom Tugenhadt and the Lib Dem’s Luciana Berger were among those to post selfies from inside the chamber demanding MPs return to action
What happens if Mr Johnson loses a vote of no confidence?
Convention suggests that he should resign as PM.
But Downing Street has suggested before that even if Mr Johnson did lose a confidence vote he would not walk away and would instead try to dissolve Parliament and force an election.
That really would be uncharted territory ad would spark a full blown constitutional crisis.
If he lost and the government falls – as it is supposed to – there would then be a 14 day period in which MPs could try to form another administration.
That could be the point at which the Remain alliance tries to put together a cross-party unity government with one objective: To delay Brexit beyond October 31 in order to avoid a No Deal split.
If no replacement administration could be formed in that two week period there would then be a general election.
John Bercow, pictured in Westminster today, responded to the Supreme Court’s ruling by saying MPs will return to work in Parliament tomorrow morning
What else could MPs do if they don’t try to oust the PM?
Trying to get rid of the PM would be a nuclear option for MPs and it would also add more uncertainty to an already volatile situation.
As a result it is thought MPs could instead keep the option of a vote in their back pockets and instead use parliamentary time to go on the attack over Brexit.
Some MPs are reportedly planning to try to force the government to publish its prorogation legal advice from Attorney General Geoffrey Cox in full.
The Remain-backing MPs know they have a majority in the Commons and could also try to reinforce the recently passed anti-No Deal law to make it ‘bullet proof’ amid suggestions the PM could try to ignore it.
Supporters of a so-called ‘People’s Vote’ will be considering whether they could use the shifting balance of power in Westminster to secure their goal.
Labour currently supports a second referendum, but only after an election.
Action could be taken against Mr Johnson over his illegal advice to the Queen, with the possibility of contempt proceedings. Theoretically this could include stripping him of his salary – or even imprisoning him at Parliament, although the latter seems very unlikely.
It has also been suggested that MPs could put Theresa May’s deal back on the table, but try to attach a ‘confirmatory vote’ or second referendum.
They could also possibly try to dictate the negotiating strategy of the government, by passing laws instructing them to seek certain terms.
What does the Supreme Court ruling mean for Brexit?
It does nothing to change the fact that the UK is still due to leave the EU on October 31.
But crucially it gets Remainer MPs back in the game. When Parliament was suspended MPs and peers were sidelined from the Brexit process.
With Parliament sitting again they will be able to challenge the government and, should they believe it is necessary, try to seize control of proceedings as they did when they passed anti-No Deal legislation.
Could Boris Johnson try to prorogue Parliament again?
Yes. The PM hinted that he could do so when he responded to the Supreme Court ruling.
When the PM first suspended Parliament he did so with the argument that he needed time to prepare a Queen’s Speech in which his new government would set out its domestic legislative plans.
That speech had been scheduled to take place on October 14 but today’s ruling puts that date in doubt.
Mr Johnson today said the government will likely try again to bring forward a Queen’s Speech but it is not immediately clear whether the PM will try to stick to the old date.
He said the Supreme Court ruling did not ‘exclude the possibility of having a Queen’s Speech’ in the near future.
Lady Hale had said this morning that a ‘normal period necessary to prepare for the Queen’s Speech is four to six days’.
That suggests Mr Johnson could try to prorogue Parliament in the first week of October in order to keep to his previous timetable.
Convention dictates that Parliament must be prorogued – and the parliamentary session formally brought to a close – before a Queen’s Speech can take place to kick off a new session.
There have been rumours swirling in Whitehall in recent days that if the court ruled prorogation was unlawful Mr Johnson could try to prorogue Parliament again but potentially for a much shorter period.
It now appears Mr Johnson is considering such a move but it would be incredibly controversial and spark fury among MPs who are likely to resist any attempt to shut down Parliament for a second time.
Jeremy Corbyn, pictured on stage in Brighton at Labour conference immediately after the Supreme Court ruling, said Mr Johnson must now ‘consider his position’
What about the Conservative Party conference?
The Tories are due to meet in Manchester next week but Mr Bercow’s decision to resume Parliament throws a grenade into their plans.
It is unclear whether the Conservatives will go ahead as planned with the four day event from Sunday until Wednesday.
It is thought ministers will try to get Parliament’s approval tomorrow for a short conference recess to allow the get together to go ahead.
But the chances of MPs voting to go back to recess immediately after Parliament’s doors have been reopened appear slim.
If the Commons rejects the proposed recess then the Tories will almost certainly have to amend their plans: The leader’s speech is due to take place on Wednesday at the same time as PMQs and Mr Johnson cannot be in two places at once.
What has the EU made of all of this?
The European Commission declined to comment today on what it described as the ‘internal constitutional matters’ of the UK.
But Brussels will be closely monitoring developments in London as Westminster tries to work out what happens next.
The last EU summit before Brexit is due to take place on October 17 and Brussels is still waiting for the UK to make a formal offer on how to break the current impasse.
The bloc will be waiting to see whether today’s chaos focuses minds or if it leads to further meltdown.