Back to Lord Advocate James Wolffe QC's submissions.

He was arguing that prorogation was not solely a political matter.

He told the court the Scottish Government could face legal action if it were to act against fundamental principles of government.

He added that prorogation maintained the government in office "without the mechanisms of accountability to Parliament" - unlike the scenario when Parliament is dissolved ahead of an election, when all MPs give up their seats.

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Lord Advocate James Wolffe QC has now finished his submission.

Next up is an oral intervention by Northern Ireland victims' campaigner Raymond McCord, represented by Ronan Lavery QC.

In his written submissions, Lord Advocate James Wolffe QC told the court Mr Johnson's decision to suspend Parliament would have "a profoundly intrusive effect" on the ability of Parliament to scrutinise the executive.

He described the decision to advise a five-week prorogation of Parliament as "an abuse of executive power which calls for the intervention of the court".

"The justification for non-interference by the courts in political questions - that such questions are a matter for executive accountability to Parliament - should not be applied to a decision which deprives Parliament of the means by which to enforce the accountability of the executive for policy or political questions," he wrote.

James Wolffe QC tells the court a "fundamental principle" of the British constitution - parliamentary accountability - is "at stake".

He tells the court the government's five-week suspension of Parliament could have "momentous consequences", with the UK set to leave the EU on 31 October.

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First up is the oral intervention by the Scottish Government, represented by Lord Advocate James Wolffe QC - Scotland's top law officer.

Lawyers for ex-PM Sir John Major will argue that current PM Boris Johnson's suspension of Parliament was motivated by "political interest".

Sir John, who was prime minister from 1990 until 1997, is among the few people given permission to intervene in the appeals.

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He will not be at the Supreme Court in person today but his lawyer, Lord Garnier, is scheduled to address the court from about 12:10 BST.

The panel of 11 justices will also hear submissions on behalf of the Welsh and Scottish governments and Northern Ireland victims' campaigner Raymond McCord.

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Dr Joelle Grogan, a lecturer in law at Middlesex University, says this is "a very difficult point in the case".

“It seems that Sir John Major is trying to lean into the politics of it - something that the courts have naturally been trying to stay away from - to ask them to look closely at motivation and intention," she says.

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Barrister and legal commentator Jeremy Brier says today will be "an extraordinary moment of legal theatre".

"We have a former Conservative prime minister [John Major] who is actually being represented by a barrister, Lord Garnier, who is a former Conservative MP, standing up in the Supreme Court arguing the current prime minister has betrayed parliamentary scrutiny," he says.

However, he says it remains to be seen how much notice the Supreme Court justices take of their intervention or how much it is "just for a bit of colour".

Meanwhile, discussions between the UK government and the EU are continuing over the terms of the UK's departure.

British Prime Minister Boris Johnson has 12 days to set out his Brexit plans to the EU, according to his Finnish counterpart.

Antti Rinne says he and French President Emmanuel Macron have agreed the UK needs to produce the proposals in writing by the end of September, or else "then it's over".

Finland currently holds the EU's rotating presidency.

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The Finnish PM intends to discuss this with the President of the European Council, Donald Tusk, and Mr Johnson in the coming days - but the position has not yet been agreed with other EU nations.

A Downing Street source said: "We will continue negotiating and put forward proposals at the appropriate time."

Read the full story here.

PM Boris Johnson prorogued Parliament earlier this month for five weeks - a decision which has been challenged in both the English and Scottish courts.

The English High Court threw out a challenge led by businesswoman and campaigner Gina Miller.

But the Scottish Court of Session ruled the PM's decision to have been unlawful.

Challenges to both these rulings are being heard in the current Supreme Court session.

On Tuesday and Wednesday we heard from lawyers representing the government as well as those for the campaigners opposing prorogation.

The government's lawyers argued the issue was a political matter - not a matter for the courts - and the prime minister was entitled to prorogue Parliament.

But lawyers arguing against the move told the court it undermined Parliamentary sovereignty and was designed to "silence" MPs ahead of Brexit.

The eleven Supreme Court justices are sitting for three days - making this the final day of the hearing.

It is not yet known when they will deliver their ruling.

However, our home affairs correspondent Dominic Casciani said yesterday that the feeling was the wait would not be long - given the importance of the issues at stake.

Image source, Getty Images

The Supreme Court is the highest court in the UK where the judges, known as justices, have the final say on the biggest legal issues.

But what power does it have to overrule the government?

BBC home affairs correspondent Dominic Casciani explains here.