Lord Keen says proroguing Parliament doesn't prevent the government and prime minister being held to account.

“It’s a fact that for a period prorogation will affect accountability in Parliament but it doesn’t prevent accountability beyond Parliament," he says.

He argues the prime minister can still be held to account by the public and the media and in upcoming party conferences.

The government's lawyer, Lord Keen, argues that campaigners who say that this prorogation is illegal are walking the judiciary into an "ill-defined minefield".

He says for judges to consider whether the government can prorogue Parliament is "forbidden territory" for the courts..

He said: "How in the context of that political minefield is the court to opine on the issue of purpose or improper purpose, or legitimate political purpose or illegitimate political purpose? How are these concepts to be defined and applied in this context?"

He adds that Parliament had the option to hold a vote of confidence in the government but that the leader of the opposition, Jeremy Corbyn, refused to do so.

He also says that Parliament has not tried to legislate on prorogation for more than 300 years.

The documents submitted to the court on behalf of Boris Johnson earlier reveal some of the possible scenarios if he loses the case and the 11 judges conclude his advice to the Queen about suspending Parliament for five weeks was unlawful.

One possibility is that the judges find it was unlawful, but their reasoning leaves open the possibility of proroguing Parliament for the same period in a lawful way.

Submitted by the government's lawyers Sir James Eadie and Lord Keen, the document says: "In that scenario, the court would and could not make any order purporting to require Parliament to be reconvened... Parliament would remain prorogued."

Image source, UK Supreme Court

Lord Keen, representing the government, says the court is being invited to "control the length of the prorogation of Parliament".

But he says the length of each Parliamentary session is regulated by constitutional convention and not by the law.

Image source, UK Supreme Court

The court is now resuming for the afternoon.

First we'll hear from Lord Keen QC, representing the government.

This morning we heard from the lawyers of those who have been given permission to intervene in the case, including former prime minister Sir John Major.

First up was Lord Advocate James Wolffe QC, on behalf of the Scottish government, who argued the prorogation was taking place at a "time-critical period" and would impact Parliament's ability to scrutinise the government.

Then Ronan Lavery QC, representing a Northern Ireland victims' campaigner, urged the court to look at the impact prorogation would have on Northern Ireland, arguing it was designed to "run down the clock" to a no-deal Brexit.

However, Supreme Court president Lady Hale made clear the court was concerned only with the prime minister's decision to prorogue Parliament, not arguments about the nature of Brexit.

Representing the Counsel General for Wales, Mike Fordham QC then argued prorogation undermined the principle of parliamentary sovereignty.

Finally we heard from Lord Garnier QC, speaking on behalf of Sir John Major.

He argued there was evidence the decision to suspend Parliament was motivated by a desire to prevent Parliament from interfering with the prime minister's policies.

Clive Coleman
BBC legal correspondent

At times it feels more like a far-fetched TV drama.

We have a prime minister of the United Kingdom who stands accused of misleading the monarch of the realm and undermining the sovereign body in our constitution – Parliament.”

Coming to assist the case against the current prime minister – the former prime minister, Sir John Major.

He is accusing Boris Johnson of having misled the Queen, suggesting that his true purpose was entirely political – to shut down debate and scrutiny in Parliament.

The government's written submission explaining what it would do if the court ruled against the prime minister has now been published.

Here's a summary of what it says:

• If the courts rule that the reason for prorogation was unlawful, but that proroguing Parliament for a similar length of time is not itself impossible, then the PM would not necessarily have to bring back Parliament before 14 October (although he might consider it)
• If the reasons were unlawful and the only available remedy in the opinion of the Supreme Court is to bring Parliament back immediately, then the PM would comply but would have to consider the timing of an earlier Queen's Speech
• If the court deems that the advice was unlawful and that prorogation never happened, the government argues there would be nothing to stop it immediately seeking another prorogation for lawful reasons
• The government argues the courts cannot act pre-emptively to stop a prorogation that hasn’t yet been granted

Shortly before midday the UK government announced it had shared with the EU a "series of confidential technical non-papers"reflecting its Brexit ideas.

It added that it would submit formal written solutions "when we are ready" rather than meeting an "artificial deadline".The EU confirmed it had received the documents shortly after.

In response, BBC's political editor Laura Kuenssberg suggested that "non-papers" could be a new word for the Brexit lexicon. She added:

Meanwhile, BBC assistant political editor Norman Smith suggested that the announcement could be a sign of Downing Street's "slowly, slowly" approach.

Here's some more from the written submission on behalf of Sir John Major.

Lord Garnier argues that because the government provided no witness statements, it remains "genuinely unclear" whether Mr Johnson disputes that he was motivated by political interests to prorogue Parliament.

He says submissions made on behalf of the prime minister's behalf "studiously avoided committing to any clear position on the issue".

Dominic Casciani
Home Affairs Correspondent

Before winding up his submissions, Lord Garnier, speaking on behalf of former PM Sir John Major, cited an important case from the New Zealand Supreme Court which was about a dodgy estate agent who misled a client.

He made the point that Prime Minister Boris Johnson had a duty to be at least better than that.

He added that he did not believe that documents given to the court provided the true reason for prorogation.

He said that in those circumstances it would be justifiable for the court to infer his true intentions.

Image source, UK Supreme Court

Lord Garnier has now finished his oral intervention and the court has adjourned until 14:00 BST.

Lord Garnier said prorogation did not just stop Parliament from sitting, but prevented it from carrying out other functions - including its ability to find people in contempt.

He told the court people who have been found in contempt of Parliament in recent years included Mr Johnson's senior adviser, Dominic Cummings.

Lord Garnier has been outlining reasons why the judiciary should be prepared to rule that this prorogation is unlawful.

In his written submission he also outlined examples of what - without judiciary control - a prime minister might be able to do.

Lord Garnier says government documents do not explain why the prorogation should be five weeks long.

He says "the inference must be that there must be some other factor” affecting its decision.

Dominic Casciani
Home Affairs Correspondent

A government submission to the Supreme Court has told the 11 justices in the prorogation case that if they rule against the prime minister he may just simply take a new decision to immediate close down Parliament again – thereby stopping it from sitting.

The submissions – which the government has so far refused to give the media - set out No 10’s legal-thinking on what the Supreme Court should do, if it were to rule that the prime minister acted unlawfully by shutting Parliament for five weeks.

In the documents Sir James Eadie QC, for the prime minister, writes that if the justices quash the original formal order to prorogue Parliament, taken on 28 August, that would mean that Parliament would remain “in session”.

In practice that would mean MPs could immediately return to the House of Commons.

He continues: “However, depending on the court’s reasoning it would still either be open or not open to the prime minister to consider a further prorogation.”

Image source, PA Media

The document – which is supposed to help the justices come up with a clear order in the event that the prime minister loses – goes on to warn the court that “there would be considerable practical uncertainty” over whether Parliament could actually reconvene – and who would be in charge of the recall.

Lord Garnier says the question to be addressed is whether prorogation causes a "material interference".

He argues that if prorogation is for a "very short period" or it comes at a time where there's no critical legislation, "there is no material interference".