If you're just joining us, here is a quick catch-up of the main points to come out of today's Court of Session hearing.

• Scotland's top law officer, Lord Advocate Dorothy Bain KC, says the UK government's block on Scottish gender recognition reforms is unlawful
• She says the Court of Session has a "constitutional duty" to review the unprecedented use of a Section 35 Order by Downing Street
• The lord advocate insists the Scottish Secretary's intervention to veto the bill was politically motivated
• If the UK government is successful, Westminster "could veto practically any act of the Scottish Parliament", Dorothy Bain warns
• Scottish Secretary Alister Jack has raised concerns that it would affect the 2010 Equality Act, which applies in Scotland, England and Wales

That's it from us for today. The civil case, which is being heard by Lady Haldane, will resume at 10:00 on Wednesday with the UK government making its arguments.

We'll be back then to take you through it.

A day of wrangling over detailpublished at 16:10 British Summer Time 19 September 2023

16:10 BST 19 September 2023

Philip Sim
BBC Scotland political correspondent

We have heard a snippet of the UK government’s key argument, with David Johnston KC beginning slightly ahead of schedule.

He essentially argues that this is just how the devolution settlement was set up; Section 35 was written into the Scotland Act for a reason.

His point is that this very scenario was foreseen by those who drafted the Act, even if it did take 24 years for it to crop up.

The KC is also keen to stress that the court’s role is to review the decision of the Scottish Secretary to veto the bill – whether it was a reasonable decision, rather than whether it would make the same decision.

His preferred definition is that the decision was “within the range of reasonable responses” given the information available.

Expect much more wrangling over definitions and legislative phrasing when the court resumes tomorrow.

David Johnston KC states that this case is not about a policy disagreement between the Scottish and UK governments.

Rather, he says, the case is concerned with the impact of this Scottish government policy on equality law in other parts of Great Britain.

Mr Johnston adds that Section 35 is "integral to the constitutional distribution of power in the Scotland Act", adding that it's an "express recognition" that competent devolved bills might have an adverse impact on operation of reserved law, and the UK government should have some limited power to intervene if that happens.

Image source, faculty of advocates

With the lord advocate concluding her submissions slightly less than half an hour before the end of today's session, Lady Haldane offers the UK government the chance to begin its case.

The Office for the Advocate General is being represented by David Johnston KC, Christopher Pirie KC and Megan Dewart.

Mr Johnston gets straight to the point and urges Lady Haldane to refuse the Scottish government's petition and adopt the "No" argument.

Dorothy Bain KC, the lord advocate, has finished addressing the Court of Session.

She has spent about four-and-a-half hours setting out her case as to why the UK government blocking the Scottish government's gender reforms was unlawful.

She urges the court to conclude that the Section 35 order should be reduced.

Image source, Getty Images

A submission by the LGBT+ charity Stonewall is being highlighted by Dorothy Bain.

The lord advocate says she agrees with the group who insist that "there is no cogent evidence... that the proposed reforms would have adverse effects on the quality of laws" in the way the Scottish Secretary asserted.

She says Stonewall go on to say "no supporting detail" has been given to back up the UK government's arguments that decision makers will not always be considering the impact on biological women as a distinct disadvantaged group compared to the impact on biological men.

James Cook
Scotland Editor, BBC News

We are in uncharted territory in the Court of Session.

This case revolves around the first ever use of part of the Westminster law which established devolution.

Section 35 of the Scotland Act 1998 allows the Scottish Secretary to veto an act of the Scottish Parliament if it modifies the law as it applies to matters reserved to Westminster and if he “has reasonable grounds to believe” that the new act “would have an adverse effect on the operation of the law as it applies to reserved matters”, in this case the Equality Act 2010.

But just because this case is unique doesn’t mean there is no precedent to discuss. Lawyers will always find a way!

For example, the Lord Advocate, Dorothy Bain KC, acting for Scottish ministers, has repeatedly referred to a 1977 case involving local government in the Greater Manchester borough of Tameside, external.

That case produced what is known as the “Tameside Duty” which, in short, requires a decision-maker to “take reasonable steps to acquaint himself with the relevant information” before making a decision.

Ms Bain’s case is that the Scottish Secretary Alister Jack presented a partisan case against the Gender Recognition Reform Act, shutting his eyes to the arguments in favour of it, and thus failed to take those reasonable steps.

Dorothy Bain continues to challenge the strength of argument behind the Scottish Secretary's decision to invoke the Section 35.

She says Alister Jack's case that the Scottish government's bill would have an adverse effect on the operation of the law has "no evidential basis".

Teasing out the arguments is the judge's real jobpublished at 15:00 British Summer Time 19 September 2023

15:00 BST 19 September 2023

Philip Sim
BBC Scotland political correspondent

There are points where these hearings may seem to be straying into the weeds.

Discussion of whether the Tameside Duty on decision-makers should be applicable, or whether or not a statement of reasons should be considered alongside the reasons outlined in the order – it can be quite difficult to follow for those watching at home, and indeed in court.

Remember that the judge has already pored over written arguments from both sides. Lady Haldane is well aware of the core points in advance.

That’s why she’s picking up on particular points of detail, and teasing out various background assumptions for examination.

Don’t assume she’s querying points because she’s unconvinced by them, either.

Stress-testing the arguments from both sides is essentially the whole job of the judge in these hearings.

Image source, Getty Images

Gender recognition is devolved - that means Holyrood can make changes to the law in Scotland.

But there are concerns from UK ministers that some of the consequences of doing that could impact wider equality legislation, which is "reserved" to Westminster (meaning this area is outside of the Scottish Parliament's competence).

However, Scottish ministers insist that their gender reforms have no impact on equality law.

In December, a High Court judge ruled that a Gender Recognition Certificate changes someone's legal sex for the purpose of the Equality Act (2010).

This law is designed to protect people from being discriminated against because of certain characteristics, which include sex or gender reassignment.

There are exceptions where certain spaces can exclude transgender people if there is are "proportionate and justified reasons" for doing so.

• What are the sticking points of the gender recognition bill?

Image source, PA Media

As we have been reporting, the UK government used Section 35 of the Scotland Act to prevent the gender recognition reform bill receiving royal assent.

Scottish Secretary Alister Jack raised concerns that it would affect the 2010 Equality Act, which applies in Scotland, England and Wales.

We'll hear from the UK government's law officer on Wednesday, with today being set aside for the Scottish government's case.

The lord advocate goes on to make the point that divergence between Scottish law and laws in other parts of the UK is not a relevant consideration in this case - as gender recognition is a matter devolved to Holyrood.

Dorothy Bain says Westminster has determined that divergence is acceptable.

She argues that the divergence with regard to gender recognition is irrelevant and the Scottish Secretary's argument is therefore irrational.

Gender recognition is a subject which the UK government has previously permitted the Scottish government to legislate on, says the lord advocate, again questioning the Scottish Secretary's intervention on this occasion.

Remember, much of the Scottish government's argument in this case is that the UK government's stance is politically motivated rather than a legal issue.

The lord advocate says the Scottish Secretary's veto on the bill is "in no way a balanced consideration of the issues".

Dorothy Bain says many of the reasons given for using Section 35 of the Scotland Act are "either unfounded, too speculative or insufficiently cogent and material to justify the exceptional step of making the order".

Lord Advocate Dorothy Bain KC is now back on her feet as the hearing resumes after lunch.

Image source, PA Media

Before breaking for lunch, Scotland's top law officer said the UK government's block on Scottish gender recognition reforms was unlawful.

Lord Advocate Dorothy Bain KC said the Court of Session 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.

If you're just joining us, here's a quick recap of what today's events at the Court of Session in Edinburgh are 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.

The court has now paused for lunch.

Dorothy Bain KC will continue her argument at 14:00.

Until then, let's take a look at her role as the lord advocate.

The office of lord advocate dates back to the 1400s, and after 1707 was the chief legal advisor to the UK government on Scottish legal matters.

After devolution, the lord advocate became legal advisor to the Scottish government, while a new office of Advocate General for Scotland was set up to advise the Crown and UK ministers on matters of Scots law.

The lord advocate tends to represent the Scottish government in big court cases, with other recent examples including the various legislative disputes over Brexit and the powers of the Scottish Parliament.

As well as advising ministers on legal wrangles and legislative plans, the lord advocate can take part in the weekly meetings of the first minister's cabinet.

They can also sit on the government benches at Holyrood and make statements to MSPs or answer questions on behalf of ministers.

The lord advocate is also the head of Scotland's independent prosecution service, the Crown Office and Procurator Fiscal Service.

• What are the roles of Scotland's lord advocate?

Image source, PA Media

A small group of protesters who oppose the Scotttish government reforms were pictured outside the Court of Session ahead of proceedings.

Three people from the Scottish Feminist Network posed with placards outside of Parliament House.

Scottish government sets out key argumentspublished at 12:05 British Summer Time 19 September 2023

12:05 BST 19 September 2023

Philip Sim
BBC Scotland political correspondent

On the first morning of the case, we’ve heard the two key pillars of the Scottish government’s argument, based on the two tests set out in the Scotland Act.

One is that the gender reform bill has not made any “modifications of the law as it applies to reserved matters”. The Lord Advocate says because the bill only changes the process for getting a gender recognition certificate, not the actual effect of the certificate itself, there is no impact on UK-wide equalities laws.

And she will come on to flesh out the second point - that the UK government needed very good reasons to veto the bill, and that those offered are “insufficient in law to justify the making of the order”.

There will be much picking over the meaning of individual words and the exact detail of the law, but these are the points on which the whole case will turn.