Image source, UK Supreme Court

Lord Pannick insists the "scope and extent of prorogative power is a matter for the courts".

“I’m not challenging the existence of a power to prorogue," Ms Miller's lawyer tells the court.

"I recognise that any prorogation involves a removal for that length of time of the powers of Parliament to scrutinise.

“But given the junior role of the executive, our case is that it’s implicit in the power to prorogue that it will not be used in such a way as to remove Parliamentary scrutiny other than so far as is reasonably necessary to accomplish a legitimate objective - and here the objective that is presented is to end one session and start another with a Queen’s speech.”

Image source, Reuters

The PM's official spokesman has said the Supreme Court case was mentioned by Boris Johnson during Tuesday's cabinet meeting.

"He mentioned the fact that the court case was ongoing and that we are confident in our arguments," the spokesman said.

Asked whether Mr Johnson should either give evidence or submit an affidavit to the court, the spokesman said: "I am not going to comment on an ongoing court case.

"The way court cases begin is that one side will set out their position and then there will be a response for the government.

"The court is the right forum for this to take place."

Dominic Casciani
Home Affairs Correspondent

An important question from Supreme Court judge Lord Hodge, who asks: Can the PM use prerogative power to prorogue Parliament in pursuit of “political advantage”?

Lord Pannick QC - working for Gina Miller - argues a prime minister cannot shut down Parliament for political advantage if it means shutting down scrutiny in the current manner.

"The prime minister cannot [as a matter of law] have discretion as to the breadth of the powers that he enjoys,” says Ms Miller's lawyer.

Lord Pannick is now turning to the issue of "justiciability" - whether the issue is a matter for the courts.

Ms Miller's lawyer says this case raises the legal question of "whether it is an invalid use of the power for the prime minister to exercise it in order to avoid parliamentary scrutiny or with the inevitable effect of avoiding parliamentary scrutiny”.

Brought by businesswoman Gina Miller - who is also leading one of today's appeals - the landmark case was about whether the prime minister or Parliament should trigger Article 50 to start the process for leaving the EU.

The case was heard in the Supreme Court after the High Court ruled Parliament must vote on whether the UK could start the process of leaving the EU.

The Supreme Court dismissed the government's appeal in the case, meaning Parliament was required to give its approval before official talks on leaving the EU could begin.

The government argued it had the power to begin negotiations without Parliament's prior agreement.

This morning, two Supreme Court justices asked Ms Miller's QC what laws have been lost as a result of Parliament's prorogation.

The BBC's political research unit have already compiled a list of those dropped laws, which you can read here.

Image source, Rosie Duffield MP

Dominic Casciani
Home Affairs Correspondent

We’ve reached a really important point in the case this today – the question of who has greater constitutional authority: Parliament or the PM?

These kinds of questions are of such importance that a certain King Charles I, when monarchs still held executive power, lost his head over the issue 370 years ago.

Lord Pannick tells the justices: “Ministers are constitutionally the junior partner - and the real issue in this case is whether the junior partner may lawfully remove the scrutiny of his activities by the senior partner – which means Parliament.”

He says the law accepts that Parliament's sovereignty would be breached were a PM to ignore an Act of Parliament. Therefore - and this is an important line of legal attack - it is an even more serious breach of that Parliamentary supremacy if a prime minister prevents MPs from holding him to account in the first place.

“The inferior body, the junior body, cannot use its constitutional powers to prevent the superior body from performing its constitutional functions particular when he constitutional function is the scrutiny of the junior partner itself.”

Image source, UK Supreme Court

Lord Pannick says the "basic principle" of the UK constitution is that “Parliament is supreme”.

He argues that to remove Parliament's ability to legislate is a "breach of parliamentary sovereignty".

"Parliamentary sovereignty is not confined to cases where the executive frustrates an Act of Parliament - it must also apply to cases - and this is a unique case - where the executive takes a decision with the purpose or effect of removing the ability of Parliament to legislate."

Dominic Casciani
Home Affairs Correspondent

Quoting from the outcome of Gina Miller's successful challenge as to whether the prime minister or Parliament should trigger Article 50, Lord Pannick has set out the UK's constitutional settlement.

In essence, when a minister uses their executive, or prerogative, powers they remain answerable to Parliament.

Lord Pannick says: "Ministers are constitutionally the junior partner - and the real issue in this case is whether the junior partner may lawfully remove the scrutiny of his activities by the senior partner [Parliament].”

The parties involved in the appeals have also provided their arguments in written form, which you can read here., external

Image source, UK Supreme Court

Lord Pannick says the difficulty in this case is the lack of precedent.

There is an "absence of occasions in which prerogative powers have been used with the purpose or inevitable effect of undermining a basic constitutional principle, that is here parliamentary sovereignty," he says.

"That has not occurred."

Ms Miller's lawyer has been citing a case from 1968 in which the ruling judge was "not deterred by precedent", he says.

Lord Pannick says that this court should also not be deterred by a lack of precedent - and instead proceed by principle.

Dominic Casciani
Home Affairs Correspondent

This is interesting.

Lord Pannick has been focusing on the material effect of proroguing Parliament and the interference with its sovereignty.

Responding questions by Lady Hale and Lord Carnworth about which bills have been lost indicate the justices may be considering the effect of prorogation.

So not just the PM's "intention" by suspending Parliament.

Image source, Getty Images

The 11 judges of the Supreme Court will rule on whether the PM's decision to suspend Parliament was legal.

But what exactly is the Supreme Court and what powers does it have?

Read our simple guide here

Image source, UK Supreme Court

Two judges, Lord Carnworth and Lady Hale, ask about which bills - proposals for new laws - were dropped when Parliament was prorogued. (We list some of them here if you're interested).

Lady Hale, the president of the Supreme Court, says an "important part of prorogation is that bills are lost" and "it would be of great interest to know" which bills were lost.

Lord Pannick, the lawyer for Gina Miller, replies: "We will find out the answer."

Image source, UK Supreme Court

The hearing's been going on for just over an hour now.

Lord Pannick, acting for Gina Miller, tells the 11 justices: "At its very lowest, the prime minister's decision to prorogue for a period as long as five weeks, in effect prevents Parliament from performing its scrutiny functions over the activities of the legislature.

"It means Parliament cannot legislate and it cannot inform itself for the purposes of legislating by the asking of parliamentary questions.

"All at a time when we say it's self-evident that Parliament will or may wish to carry out such activities."

Dominic Casciani
Home Affairs Correspondent

Here's some further detail on Lord Pannick's focus on a lack of witness statement from the PM.

Lord Pannick says the Supreme Court should draw an inference about the PM's motives because of the absence of material setting out what he was up to.

He suggests the PM would not have recommended a five-week prorogation were it not for his desire to close down Parliamentary debate of Brexit.