Image source, Getty Images

In a statement after the ruling, James Libson, executive partner at Mishcon de Reya, which represented Gina Miller in the case, said: "We are glad that the court recognised the threat to the rule of law caused by a prorogation based on misleading advice given to the Queen.

"This second success for our client Gina Miller in the Supreme Court is a testament to her resolve to take whatever steps are required to ensure executive overreach does not become a feature of our democracy.

"This case shows that our courts can be relied on to hold the executive to account when necessary and is evidence of the robustness of our system of separations of powers."

Image source, UK Supreme Court

Delivering the unanimous verdict of the 11 Supreme Court justices, Lady Hale, the president of the court, said: "This court has already concluded that the prime minister's advice to Her Majesty was unlawful, void and of no effect.

"This means that the Order in Council to which it led was also unlawful, void and of no effect and should be quashed.

"This means that when the Royal Commissioners walked into the House of Lords it was as if they walked in with a blank sheet of paper.

"The prorogation was also void and of no effect. Parliament has not been prorogued.

"This is the unanimous judgement of all 11 justices."

Clive Coleman
BBC legal correspondent

Wow! This is legal, constitutional and political dynamite.

It is worth just taking a breath and considering that a prime minister of the United Kingdom has been found by the highest court in the land to have acted unlawfully in shutting down the sovereign body in our constitution, Parliament, at a time of national crisis.

The court may have fallen short of saying Boris Johnson had an improper motive of stymieing or frustrating parliamentary scrutiny, but the damage is done, he has been found to have acted unlawfully and stopped Parliament from doing its job without any legal justification.

And the court has quashed both his advice to the Queen and the Order in Council which officially suspended Parliament.

That means Parliament was never prorogued and so we assume that MPs are free to re-enter the Commons.

This is the most dramatic example yet of independent judges, through the mechanism of judicial review, stopping the government in its tracks, because what it has done is unlawful.

Be you ever so mighty, the law is above you – even if you are the prime minister. Unprecedented, extraordinary, ground breaking.

It is difficult to overestimate the constitutional and poltical significance or today’s ruling.

Image source, AFP

"The ruling today speaks volumes," says campaigner and businesswoman Gina Miller, one of those who brought the legal challenge against the government.

"This prime minister must open the doors of Parliament tomorrow. MPs must get back and be brave and bold in holding this unscrupulous government to account."

Liberal Democrat leader Jo Swinson says the prime minister "misled Queen and country".

"This confirms what we already knew – Boris Johnson isn’t fit to be prime minister," she says.

"He’s misled Queen and country, and unlawfully silenced the people’s representatives. I’m on my way to resume my duties in the Commons and stop Brexit altogether."

Shadow Brexit Secretary Keir Starmer praises the "strong judgement" from the court.

"Strong judgement from the court proves Boris Johnson has no regard for the law. Yet again he has been found out," he says.

"The sooner we resume our work challenging and defeating him the better."

Dominic Casciani
Home Affairs Correspondent

To be clear: the Supreme Court upheld the Court of Session in Edinburgh's judgment, which said that Boris Johnson had "an improper purpose" when he advised The Queen to prorogue Parliament for five weeks.

That means the Scottish court's judgement that he misled The Queen stands.

Image source, EPA

Reacting to the ruling, Speaker of the House of Commons John Bercow says: "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."

Image source, Getty

"I'm delighted the Supreme Court has stopped this unconstitutional act in its tracks," says former attorney general Dominic Grieve.

He adds that it is "perfectly obvious" that the reasoning for the prorogation was "bogus" and that the prime minister should reflect on the "untold damage" of his approach on British institutions.

Dominic Casciani
Home Affairs Correspondent

This is the worst-possible outcome for the prime minister.

Gasps in court, even applause.

The Supreme Court has published its full judgement, external on its website here.

The full text of Lady Hale's summary is also available here, external.

"The prorogation was void and of no effect," Lady Hale says. "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".

It is for Parliament to decide what to do next, she says.

Boris Johnson’s decision to suspend Parliament for five weeks was unlawful, the Supreme Court says.

"The effect on the fundamentals of our democracy was extreme," Lady Hale says.

"No justification" has been put before the court, she adds.

Lady Hale says "this was not a normal prorogation" and it "prevented Parliament from carrying out its duties" in the crucial period before the Brexit deadline.

The Supreme Court has overturned the ruling from the English High Court and determined that it can rule on whether the suspension was lawful.

Lady Hale says the decision is unanimous.

The prime minister has lost the first stage but not necessarily the whole case.