The Supreme Court has been hearing two appeals to determine whether the PM acted lawfully in suspending Parliament
Eleven Supreme Court justices are sitting for three days, led by Lady Hale
Lord Keen QC, Advocate General for Scotland, spoke for the UK government
Earlier, the court heard from Lord Pannick QC, for Gina Miller, who is challenging the suspension
The five-week suspension of Parliament began a week ago
MPs are not scheduled to return until 14 October
Opposition parties have called for Parliament to be recalled
Image source, UK Supreme CourtLord Keen, the lawyer acting for the government, says it is not for the courts to "cross the boundaries and intrude upon the proceedings of Parliament".
But he adds: "If this court finds that the advice of the prime minister was unlawful, the prime minister will take the necessary steps to comply with any declaration made by the court."
Image source, UK Supreme CourtLord Keen is the Advocate General for Scotland - the lawyer acting for the UK government in the appeal against the Scottish court's decision that Boris Johnson's suspension was unlawful.
Lord Keen starts by citing previous legal judgements showing there is no difference between Scottish and English interpretations of the law.
Referring to the conflicting rulings which came from the English High Court and Scottish Court of Session over whether Mr Johnson's suspension of Parliament was lawful, he tells the judges "it is quite clear" that the decisions "did not result from any substantive difference between English and Scottish law".
BBC News Channel
Alison Young, a professor of public law at the University of Cambridge, says she expects the government's lawyer, Lord Keen, to argue that the case "isn't about law, it's about politics".
She says a probable argument will be that there "are no legal limits on how you can prorogue Parliament... and even if there were legal limits, they weren't breached here".
Image source, UK Supreme CourtLord Keen is the lawyer for the UK government who is trying to overturn the Scottish court ruling that the suspension of Parliament was unlawful.
BBC News Channel
One of those bringing a legal challenge against Boris Johnson's decision to suspend Parliament, the SNP MP Joanna Cherry, says she is "cautiously optimistic" that the Scottish Supreme Court's decision will be upheld by the UK Supreme Court.
"If they don't then they will be accepting that it is possible under the constitution for the prime minister of a minority government to shut down Parliament if it is getting in his way, and that just can't be right," she tells the BBC News Channel.
"I think the Scottish court's ruling will prove decisive and persuasive," she adds.
If the Supreme Court says Boris Johnson's decision to suspend Parliament was illegal, Ms Cherry says she expects MPs to immediately return to the Commons.
Image source, EPAShe's been at the forefront of two key legal battles over Brexit - but who is Gina Miller?
Image source, UK Supreme CourtThe 11 judges have returned to the courtroom following an hour's lunch break.
The Supreme Court has seen huge traffic on its website today.
Its live streaming service usually is accessed about 20,000 times a month but this morning, the live stream was accessed 4.4 million times.
In the hour before the 13:00 BST break for lunch, 2.8 million stream requests were logged.
Next up, the judges move on to the government's appeal against the ruling by Scotland's highest court, the Court of Session.
Earlier this month, the Scottish court ruled that Boris Johnson's suspension of Parliament was unlawful. It was a victory for a cross-party group of politicians who had challenged the PM's suspension.
Now, the government is appealing against the Scottish court's ruling.
Representing the UK government is Lord Keen, who will be starting his submission at 14:00 BST.
Practising lawyer Bobby Friedman believes the matter is a political matter, and so the High Court's ruling should be upheld.
Asked what he'd say if he was in front of the judges, he asks why it was acceptable to prorogue Parliament for three months in 1930, but is unacceptable to do it for 24 days today.
"Why was it acceptable to prorogue Parliament to pass the Parliament Act in 1949 but not acceptable to do it today," he adds.
He says: "In order to say there's an abuse, an improper use of this power, you have to ask the court to make a political judgement about what is or what isn't acceptable.
"That isn't what our judiciary should be doing.
"That's a matter for our executive and they can be held to account at elections and by public opinion and indeed by Parliament."
He says judges should not be put "at the forefront of this political issue".
As well as protesters, there is a crowd of journalists outside the Supreme Court in Parliament Square, Westminster.
Image source, Getty Images
Image source, Getty Images
The World at One
BBC Radio 4
Dr Joelle Grogan, a senior law lecturer from Middlesex University, says the easiest way to look at the case is at its "three core arguments".
She says: "The first issue is on justiciability, which boils down to, can the courts be asked this question and can they answer this question? Is this question so political, that the courts have nothing to do with it?
"The second issue would be legality. Is there anything we can see in our law, in our legal system... against which we can measure this advice of the prime minister to the Queen to prorogue Parliament?
"The third issue is the most difficult to establish. Is the lack of evidence given by the prime minister for his reasons to prorogue a way in which the court can look into that silence."
She adds that while prorogation is a completely ordinary power, because Boris Johnson has not provided any reasons why he has prorogued for such a long time, "should we infer from that that you had an improper motive".
Legal commentator Joshua Rozenberg tweets...
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Dominic Casciani
Home Affairs Correspondent
Inside court, it has come down to one simple question: who is supreme? Is it the prime minister or Parliament?
Lord Pannick QC acting for Gina Miller, the anti-Brexit campaigner, effectively says that either by his attention or his effect, Boris Johnson has silenced Parliament and its ability to scrutinise the executive.
And that, he said, is a huge breach of the UK's constitution.
The great big blonde invisible elephant in this room is the prime minister himself because we learnt this morning that he hasn't prepared a witness statement for the 11 justices here in court explaining why he advised the Queen to prorogue - or suspend - Parliament.
Lord Pannick says it is the absence of that witness statement which, he argues, shows the prime minister is trying to hide what he was really up to.
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BBC Radio 4
"Arguably this is the most dramatic constitutional law case since the bill of rights in 1689 established our current system of parliamentary sovereignty," says the BBC's legal correspondent Clive Coleman.
"Let's just remind ourselves - a prime minister of the United Kingdom stands accused of misleading the monarch and then undermining the sovereign body in our constitution, Parliament.
"This has all arrived through appeals now at the highest court in the land against an incredibly tight timeline at a time of national crisis.
"This really couldn't be any bigger."
Meanwhile, away from the Supreme Court, the leader of Plaid Cymru has said the Welsh party should campaign to "cancel" Brexit if a general election is called.
Adam Price said members will be asked to back a change in the party's current position of supporting a further EU vote.
Read more here.
Gina Miller's lawyer Lord Pannick has now finished making his case challenging the prorogation of Parliament.
Here's a recap of his main arguments:
We're coming to the end of the first session now.
Lord Pannick is due to finish speaking at 13:00 BST, although he may run slightly over.
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Image source, Supreme CourtLord David Pannick QC is a barrister in the UK and crossbencher in the House of Lords.
He practises in a broad range of areas with a particular focus on public law, human rights and constitutional law.
He has acted in more than 20 cases in the Supreme Court since its creation in 2009, more than 25 cases in the European Court of Justice and more than 30 cases before the European Court of Human Rights.
His clients have included the Queen, for whom he won an injunction to halt allegations about her home life by a reporter who had gained employment as a footman.
He has also acted in a large number of public law cases of the last 25 years, including successfully establishing in the House of Lords the legality of the whole life tariff for Myra Hindley, the Moors Murderer.
Lord Pannick is acting today on behalf of campaigner Gina Miller - who brought one of today's appeals.
Barrister tweets...
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