The prime minister suspended – or prorogued – parliament for five weeks earlier this month, saying it was to allow a Queen's Speech to outline his new policies.
But the Supreme Court today ruled it was wrong to stop parliament carrying out its duties in the run-up to the Brexit deadline.
The court's president, Lady Hale, said: ‘The effect on the fundamentals of our democracy was extreme.’
She said the unanimous decision of the 11 justices was that Parliament had not been prorogued – the decision was null and of no effect - and it was for the Speakers of the Commons and Lords to decide what to do next.
Lady Hale said: ‘It is important once again to emphasise that these cases are not about when and on what terms the United Kingdom is to leave the European Union.
‘They are only about whether the advice given by the prime minister to Her Majesty the Queen... was lawful.’
Lady Hale said the case is a ‘one-off’, having come about ‘in circumstances which have never arisen before and are unlikely to ever arise again’.
She added the court found the issue was ‘justiciable’ – capable of challenge in the courts.
Lady Hale told the court that ‘a decision to prorogue, or advise the monarch to prorogue, will be unlawful if the prorogation has the effect of frustrating or preventing without reasonable justification the ability of parliament to carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive’.
She continued and said: ‘The court is bound to conclude therefore that the decision to advise Her Majesty to prorogue was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions.’
Lady Hale said the prorogation was ‘void and of no effect’, adding: ‘Parliament has not been prorogued.’
Lady Hale continued that the speakers of the Houses of Commons and Lords ‘can take immediate steps to enable each house to meet as soon as possible’.
In a statement, Speaker John Bercow said: ‘I welcome the Supreme Court's judgement that the prorogation of Parliament was unlawful.
‘The judges have rejected the government's claim that closing down Parliament for five weeks was merely standard practice to allow for a new Queen's Speech.
‘In reaching their conclusion, they have vindicated the right and duty of Parliament to meet at this crucial time to scrutinise the executive and hold Ministers to account.
‘As the embodiment of our parliamentary democracy, the House of Commons must convene without delay. To this end, I will now consult the party leaders as a matter of urgency.’
While Labour leader Jeremy Corbyn said the Supreme Court ruling showed Mr Johnson had ‘acted wrongly in shutting down parliament and demonstrates a contempt for democracy and an abuse of power’.
‘I invite Boris Johnson to consider his position,’ he said.
Former defence secretary Penny Mordaunt backed showed her support for the PM on Twitter.
The Portsmouth North MP said: ‘We’re testing our institutions, but they are strong and will see us through this. Democracy doesn’t die if a PM attempts to prorogue Parliament, or if the #SupremeCourt overrules him, or if MPs act on their consciences. It will if we don’t respect the referendum and each other’
Protesters outside the Supreme Court erupted into cheers following the ruling.
Chants of ‘re-open parliament’ and ‘Johnson out’ quickly began as the news spread through crowds, who were watching livestreams on mobile phones outside.
Former Green Party leader Caroline Lucas has said the Supreme Court's decision is ‘just the start’.
‘Supreme Court decision has stopped Johnson in his tracks,’ she tweeted. ‘Parliament cannot be swept aside by prime ministerial whim. It must resume immediately.
‘This is just the start. Our democracy won't be safe until we have a written constitution, protecting our rights and rule of law.’
Labour's shadow Brexit secretary Keir Starmer said: ‘Strong judgement from the court proves Boris Johnson has no regard for the law. Yet again he has been found out. The sooner we resume our work challenging and defeating him the better.’