The UK Government's most senior Scots lawyer has rejected Scottish Government claims that Alister Jack acted “irrationally” when he blocked Holyrood's Gender Recognition Reform Bill.

In legal arguments published on Friday afternoon, Lord Stewart, Advocate General for Scotland, said that for ministers in Edinburgh to back up their claim, they would need to prove Mr Jack had acted in a manner which was “so outrageous in its defiance of logic or of accepted moral standards that no sensible person who had applied his mind to the question to be decided could have arrived at it”.

READ MORE: Alister Jack decision to veto Holyrood gender reforms 'irrational'

The Gender Recognition Reform Bill —  passed by MSPs just before Christmas —  simplifies the process for obtaining a gender recognition certificate by removing the need for a medical diagnosis of gender dysphoria.

It also reduces the length of time someone would need to live in their acquired gender from two years to less than 12 months and lowers the minimum age for applications from 18 to 16. 

In January, Mr Jack made an order under section 35 of the Scotland Act to block the legislation from gaining Royal Assent.

It was the first time in the history of devolution that the veto has been used. 

In his statement of reasons, Mr Jack warned that having self-ID in Scotland but requiring some form of medical safeguarding in England, Wales or Northern Ireland would impact the UK-wide Equality Act, and create "two parallel and very different regimes" for issuing and interpreting Gender Recognition Certificates.

In April, Humza Yousaf confirmed he would mount a legal challenge.

During the SNP leadership contest, he was the only one of the three candidates to commit to fighting the block in the courts.

He warned that not doing so could risk more Holyrood laws being blocked, undermining devolution itself.

The Scottish Government's legal challenge is due to be heard over three days at the Court of Session in Edinburgh next month.

READ MORE: EXPLAINED: Section 35 Scotland Act: What is it and why is Alister Jack using it?

In their petition for a judicial review, the Scottish Government said their argument for overturning the veto was based on four counts.

They claimed the Tory minister had made a “material error of law”, that his concerns about the safeguards in the Bill were “irrelevant” to the section 35 order, and that his reasons were “inadequate”, which would make the order “unlawful”.

The submission read: “Having regard to the absence of any supporting evidence produced by the Secretary of State, and in the context of research, consultation and comparative information available to, and considered by, the Scottish Parliament during the Bill’s passage, the Secretary of State’s concerns about the operation of the Bill are irrational”.

The Scottish Government's lawyers also said Mr Jack needed to show why he had “reasonable grounds” to believe the Bill would impact on the operation of reserved laws.

READ MORE: SNP ministers had 'no option' in launching gender reform legal fight

In the newly published note of the UK Government's argument, the Advocate General states that the Scottish Secretary did "not require definitive empirical data as to the likely adverse effects of the provisions in the Bill."

Instead, he should be able to "take account of the predictions of the Scottish Government as to the effects of the provisions... and to apply logic to whether there are likely to be adverse effects on the law as it relates to reserved matters."

Lord Stewart also argues that the Scottish Government has "misinterpreted" Mr Jack's reasons for blocking the Bill.

"It is not the fact of the divergence itself which is considered by the Secretary of State to be adverse, but rather the effect that the particular examples of divergence would have on the operation of the law as it applies to reserved matters," he writes.

He also said that it was wrong for the Scottish Government to class Mr Jack's concerns as a "policy disagreement."

"Even if the Bill is within the competence of the Scottish Parliament if it has an adverse effect upon the operation of the law as it applies to reserved matters in the predictive and evaluative judgment of the Secretary of State," Lord Stewart added.

He said concern about "the adequacy of safeguards in the Bill had been expressed during the passage of the Bill through the Scottish Parliament, including by the UN Special Rapporteur."

The lawyer also argues that section 35 is part of the constitutional framework of devolution and that the conditions for triggering it have been met. 

“It forms part of a suite of carefully crafted checks and balances on the devolution of law-making power from the UK Parliament to the Scottish Parliament, which includes, but goes beyond, the question of competency alone," Lord Stewart argues. 

He says a 2012 Memorandum of Understanding referring to the use of section 35 as “a matter of last resort” is not legally binding. 

Meanwhile, a number of LGBT+ organisations have been given permission to intervene in the legal challenge.

Stonewall, Gendered Intelligence and the Institute for Constitutional and Democratic Research (ICDR) will be able to present written evidence to the court on what they say will be the adverse consequences of the UK Government’s decision.