Historic court defeat for Boris Johnson

Court President Lady Hale said the effect of the prolongued suspension "on the fundamentals of our democracy was extreme".

READ MORE: Reaction following historic Supreme Court ruling as suspension of parliament deemed 'unlawful' 

She said it was now up to the Speakers of the Commons and Lords to convene parliament, and the Prime Minister did not require to do anything.

The decision by the court’s 11 justices was unanimous.

It brought immediate demands for Mr Johnson to resign.

The SNP said his position was "untenable".

Mr Johnson had claimed the prorogation, the longest outside an election since 1931, had merely been to reset the parliamentary calendar and have a Queen’s speech and new legislative programme.

However two legal challenges brought to the Supreme Court argued the true reason had been to stop parliament scrutinising his plan for a “do or die” Brexit by October 31.

The court rejected the UK Government’s argument that the issue of prorogation was wholly a matter of politics, and therefore “forbidden territory” for the court.

However court President Lady Hale said the matter was "justiciable" and the courts had jurisdiction over the limits of the Prime Minister's power.

She said it was "not a normal prorogation" and had the effect of frustrating parliament from carrying out its normal functions.

Delivering the decision, Lady Hale said: “No justification for taking action with such an extreme effect has been put before the court. 

“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.”

Turning to the legal remedy, she 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. 

READ MORE: John Bercow: Westminster must reconvene 'without delay' following court ruling 

“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 judgment of all 11 Justices.

“It is for Parliament, and in particular the Speaker and the Lord Speaker to decide what to do next. Unless there is some Parliamentary rule of which we are unaware, they can take immediate steps to enable each House to meet as soon as possible. 

It is not clear to us that any step is needed from the Prime Minister, but if it is, the court is pleased that his counsel have told the court that he will take all necessary steps to comply with the terms of any declaration made by this court.”

The ruling followed parallel legal challenges in England and Scotland.

Business woman Gina Miller, who previously secured MPs a vote on triggering Brexit, challenged the prorogation at the High Court in London.

A group of 75 peers and MPs led by SNP MP Joanna Cherry QC also brought a case at the Court of Session in Edinburgh.

Both were initially unsuccessful.

The High Court ruled prorogation was a political matter and therefore non-justiciable, and Ms Miller appealed directly to the Supreme Court. 

In Scotland, a single judge, Lord Doherty, make a similar ruling, and the campaigners then appealed to the Court of Session’s Inner House.

The three judges of the Inner House, including Scotland’s most senior judge, the Lord President Lord Carloway, ruled the matter could be considered by the courts.

They ruled that the advice to the Queen had been unlawful because it was based on an improper motive - to stymie parliamentary debate and scrutiny over Brexit.

The UK Government then appealed that ruling to the Supreme Court, where it was heard alongside Ms Miller’s appeal last week.