That's all from our live page. Here's the headlines from today:

• Judge Lady Haldane has retired to consider whether the UK government was within its rights to veto the Gender Recognition Reform Bill passed at Holyrood
• The judge is expected to take some weeks to consider her ruling.
• Earlier David Johnston KC insisted the UK government's decision to block Holyrood gender recognition reforms was in "no way unconstitutional"
• Johnston urged the court to reject the Scottish government's petition
• He rejected claims using a Section 35 Order could undermine key constitutional principles or "rub against them"

• The Section 35 Order is not a "broad or unfettered power" he added
• The UK government has raised concerns about the impact of the legislation on Britain-wide equality laws
• Scottish ministers are aiming to overturn the decision
• Lord Advocate Dorothy Bain KC said the veto was unlawful and irrational

A 'unique and very interesting and challenging' casepublished at 16:09 British Summer Time 20 September 2023

16:09 BST 20 September 2023

James Cook
Scotland Editor, BBC News

The UK government’s argument boils down to this: Section 35 of the Scotland Act is a fundamental pillar of the architecture of devolution and the Scottish secretary has every right to lean on it.

David Johnston KC called the Lord Advocate’s claim that Alister Jack had used Section 35 improperly simply to thwart policy he opposed “a red herring”.

Section 35 can only be invoked to veto an act of the Scottish Parliament if the act modifies the law as it applies to Westminster and if the secretary of state “has reasonable grounds to believe” that it “would have an adverse effect on the operation of the law as it applies to reserved matters”.

Mr Johnston insisted the bill had been vetoed on those grounds and nothing else.

This was a hearing full of discussion about what it means to be reasonable; what counts as an “adverse effect”; and what operation of the law actually involves.

Perhaps some of that might have seemed dry and technical to some observers but not to the most important person in the room.

As proceedings concluded the judge Lady Haldane called the case “unique and very interesting and challenging”.

She must now overcome those challenges to produce an opinion which will be another milestone on the devolution journey.

Analysispublished at 15:34 British Summer Time 20 September 2023

15:34 BST 20 September 2023

Philip Sim
BBC Scotland political correspondent

With arguments complete, Lady Haldane has said she will take “some time” to consider her ruling.

The final question for the court was what to do about a looming appeal in a tangentially-related case – an appeal by the campaign group For Women Scotland in a case about the Scottish government’s definition of a woman.

Both sides in this case cited precedent from that case, which oddly enough was chaired by the same judge.

They have agreed again on the next steps – that Lady Haldane should go off to start on her judgement regardless.

If the appeal ruling was to somehow come out first and throw a spanner in the works, the thinking is that the court could reconvene and hear some fresh submissions.

But everyone will likely be hoping that the judgement is done and dusted sooner rather than later.

Judge Lady Haldane gets proceedings under way again, but not for long.

She tells the court she will make "avizandum", which means "further consideration".

The court is adjourned.

The Court of Session adjourns and will reconvene at 15:15.

Analysispublished at 15:05 British Summer Time 20 September 2023

15:05 BST 20 September 2023

Philip Sim
BBC Scotland political correspondent

David Johnston has spent a good part of his time knocking down the arguments advanced by the Lord Advocate for the Scottish government yesterday.

But he has also set out the key pillars of the UK government's case.

The first is that this is simply part of the devolution settlement - ministers in Edinburgh may not like it, but the power of veto was written into the Scotland Act from day one.

And the second is that Alister Jack had "reasonable grounds" to use the veto power, because he believes the Holyrood reforms will cut across reserved law.

Mr Johnston says it's "quite simple" to draw a line from the Scottish bill through the 2004 Gender Recognition Act and on to the 2010 Equality Act - the latter of which MSPs are not allowed to modify. The Lord Advocate would, of course, disagree.

How successful those arguments are will be a matter for the judge, Lady Haldane - but it may be some weeks or months before we find out.

Just a reminder that this case is all about the Gender Recognition Reform (Scotland) Bill, external.

MSPs voted in December last year to pass the bill by 86 votes to 39.

So far, so straightforward? Not quite.

Scottish Secretary Alister Jack used UK government powers to veto the bill, saying it could interfere with equalities law across the whole country.

Scottish ministers then lodged a legal challenge at the Court of Session in a bid to overturn that decision and to have the reforms come in to law.

So this is the second day of legal argument to decide who is right and who is wrong.

We now return to Lord Advocate Dorothy Bain KC who is clarifying some points that arose from yesterday's session.

The Section 35 Order is not clear nor a good example of parliamentary drafting, says Ms Bain.

She confirms there are six-single sex schools all in the private sector and they will deal with Scottish gender recognition certifcates.

Mr Johnston says complaints that the word “may” was used in the reasoning for the Section 35 Order is not credible, since such phrasing is “inevitable” when the Scotland secretary is trying to predict potentially “adverse effects” of the bill.

He goes on to defend UK government arguments that the bill could pose problems for the tax and benefits system, single-sex schools and potentially fraudulent claims for gender recognition certificates.

He takes issue with the Lord Advocate pointing out that there are only six single-sex schools in Scotland. He says the bill could cause “wider” issues for single-sex schools elsewhere in the UK, including those near the Scottish border.

Mr Johnston finishes his submission calling on the court to reject the Scottish government's petition.

Here's a quick recap of what this case is all about.

• The Scottish government wants to make it easier for people in Scotland to change their legally-recognised gender
• MSPs approved the Gender Recognition (Reform) Bill last December by 86 votes to 39
• But it has proved hugely controversial - leading to this legal face-off between the Scottish and UK governments
• Scottish Secretary Alister Jack used UK government powers to veto the bill, saying it could interfere with equalities law across the whole country
• Scottish ministers then lodged a legal challenge at the Court of Session in a bid to overturn that decision and to have the reforms come in to law.

Johnston says the UK government has argued it is "highly problematic" to have two different gender recognition systems within the UK.

There is a concern that systems across the UK are integrated, the KC adds.

"There's a fundamental problem perceived," he says and he tells the court it would lead to a lot of work being required and have an adverse effect on the law.

Mr Johnston says: “The concern is only to see that there is a reasoned decision on the principle important issues in controversy, which I of course submit that there is.”

Namely, that the bill has an “adverse effect on the operation of the law as it applies to reserved matters”.

Image source, PA Media

Here's a quick recap of yesterday's proceedings:

Scotland's top law officer said the UK government's block on Scottish gender recognition reforms was unlawful.

The Scottish government is seeking to overturn the veto at the Court of Session in Edinburgh.

Lord Advocate Dorothy Bain said the court had a "constitutional duty" to review the unprecedented use of a Section 35 Order by Downing Street.

The UK government has raised concerns about the impact of the legislation on Britain-wide equality laws.

David Johnston KC, who is acting for the UK government, urged the court to reject the Scottish government's petition.

• Read more here
• Scottish government challenges gender reform block
• Court clash over move to block gender reforms

Mr Johnston turns to a series of points responding to the Lord Advocate's appearance in the court yesterday.

The KC, representing the UK government, argues the Scottish secretary gave substantial reasons for using the Section 35 Order, external.

He defends the adequacy of the reasons for the order and says they leave the reader in no doubt.

Much of the criticism the Lord Advocate made about the reasons was misdirected, he adds.

Judge Lady Haldane gets proceedings under way again after lunch.

David Johnston KC resumes outlining the UK government's case.

Remember if you want to follow the proceedings live just click on the play icon at the top of the page.

Image source, PA Media

We return to our live coverage as the Court of Session in Edinburgh with less than five minutes to go until proceedings get underway again. Here's a reminder of what's been happening over the last two days:

The UK government's decision to block Holyrood gender recognition reforms was in "no way unconstitutional", a judge has heard.

David Johnston KC, acting for UK ministers, said it did not violate nor "rub against" any constitutional principles and was lawful.

Scottish ministers are aiming to overturn the decision at the Court of Session in Edinburgh.

Lord Advocate Dorothy Bain said the veto was unlawful and irrational.

• Read more here
• UK block on Scottish gender law unlawful, court hears
• Court clash over move to block gender reforms

Lady Haldane adjourns the hearing until 14:00.

If you're just joining us here's a quick recap of this morning's headlines:

• David Johnston KC, who is acting for the UK government, urged the court to reject the Scottish government's petition
• Johnston rejected claims using a Section 35 Order could undermine key constitutional principles or "rub against them"
• He insisted the use of the order "is in no way unconstitutional"
• The Section 35 order is not a "broad or unfettered power" he added
• The UK government has raised concerns about the impact of the legislation on Britain-wide equality laws

Join us after lunch when we'll bring you live coverage and analysis of this afternoon's session.

Mr Johnston says the Lord Advocate’s criticisms of Mr Jack’s reasoning for making the order were “off the mark”.

He tells the court they do not comply with the accepted approach to scrutiny of reasons set out in the case law.

The lawyer says there was nothing in correspondence in the immediate aftermath of the Section 35 Order being made which suggested Scottish ministers had a “genuine failure to comprehend” why it had been done.

He denies that the Scotland secretary’s reasoning was “hypothetical and speculative”, instead describing it as “predictive judgement” to assess how the bill would be likely to impact the operation of reserved law.

Does the Scottish legislation overlap with UK-wide law?published at 12:56 British Summer Time 20 September 2023

12:56 BST 20 September 2023

Philip Sim
BBC Scotland political correspondent

We have arrived at a key moment in the UK government case – their explanation of how the Scottish legislation overlaps with UK-wide law.

David Johnston KC says it’s “quite simple”, but it’s worth unpacking at least a little.

The Holyrood bill streamlines the process for getting a gender recognition certificate, and opens it up to more people.

The UK government contend that this actually changes the definition of what a “full gender recognition certificate” is in law – because it would encompass a wider group of people.

That in turn changes the “substance” of the Gender Recognition Act, which sets out that obtaining this certificate constitutes a change of sex “for all purposes”.

And THAT in turn has a knock on effect on the 2010 Equality Act, which sets out the protections which people are entitled to on the basis of their sex. Crucially, that law is reserved to Westminster.

The Scottish government rejects this as going too far, but that is the sequence of steps which the UK government are relying on to show that reserved law has been affected.

Third relevant case for publication of sumbissionspublished at 12:47 British Summer Time 20 September 2023

12:47 BST 20 September 2023

James Cook
Scotland Editor, BBC News

A third relevant case is that of Joanna Cherry KC’s challenge to the UK government over its prorogation of parliament, in which Scotland’s most senior judge, the Lord President, Lord Carloway, discussed access to other written materials, external.

“In the absence of special circumstances,” he wrote, “they too will be open for inspection. Parties may, in accordance with past practice, assist in facilitating access to those documents by the press. The court will continue to do so.”

Lady Haldane appears to take a similar view of the importance of open justice and, with the assistance of her very helpful clerk, we do now have copies of the submissions, with some redaction of personal information.