She questioned how his decision could be described as reasonable when it was largely based on advice from inside the UK Government and groups “hostile” to the GRR Bill.
“He can’t just close his eyes to one side of the argument. There would be no point in imposing a duty on him if that was acceptable,” Ms Bain told judge Lady Haldane.
The Bill is intended to make it simpler and quicker for a person to obtain a gender recognition certificate (GRC), changing their sex in the eyes of the law.
Under the current 2004 UK law, obtaining a GRC requires a medical diagnosis of gender dysphoria, living in an acquired gender for two years, and a minimum age is 18.
The Holyrood Bill would replace medical diagnosis with self-declaration, or self-ID, cut the two year period to three to six months, and lower the minimum age to 16.
Mr Jack used an unprecedented order under Section 35 of the Scotland Act to stop the GRR BIll becoming law after MSPs passed it by 86 to 39 in December with cross-party support.
He said the Bill, although within Holyrood’s competence, could nevertheless have adverse effects on the operation of UK-wide reserved law, notably equality law.
He said the Bill would create two different parallel systems for GRCs, one of which would only have effect in Scotland, and one which had effect UK-wide.
It meant someone could be one legal sex in Scotland and another in England and Wales.
Mr Jack suggested tax and benefit IT systems may not be able to cope with that.
He also said the Bill would remove safeguards for women and girls, and that it might attract fraudulent applications and be exploited by sexual predators.
On the opening morning of a scheduled three -day hearing at the Court of Session, Ms Bain said Scottish ministers regarded Mr Jack’s decision-making as unlawful, irrational and unreasonable and the Section 35 order should therefore be “reduced”, or annulled.
easonable and the Section 35 order should therefore be “reduced”, or annulled.