The Scottish Government’s legal bid to overturn a Westminster block on controversial gender reforms is “not actually about the legislation itself”, Humza Yousaf has said.

The First Minister said the court challenge, which is expected to cost taxpayers hundreds of thousands of pounds, was about “principle” rather than the content of the Bill.

The Scottish Tories said it showed he was manufacturing a grievance at the public's expense.

Scottish and UK Government lawyers will tomorrow begin a three-day hearing at the Court of Session before Lady Haldane about the frozen Gender Recognition Reform Bill (GRR).

MSPs passed the Bill, which is intended to make it simpler and easier for people to change their legal sex, in December but Scottish Secretary Alister Jack vetoed it in January.

Mr Jack stopped the Bill becoming law by issuing an order under Section 35 of the 1998 Scotland Act, the first time such an order has been made in 24 years of devolution.

He argued that, although within Holyrood’s powers, the Bill would adversely affect how reserved UK laws operate in fiscal, social security and equal opportunity areas.

The Scottish Government argues Mr Jack’s decision was fundamentally flawed as it was based on an error in law and his reasons were irrational, irrelevant and inadequate.

Whoever wins this week’s first round, the case is expected to be appealed all the way to the UK Supreme Court for a definitive ruling, a process that could take months or years. 

If the Scottish Government's case prevails, the Bill will become law.

If the UK Government wins, the Bill will be sent back to Holyrood for "reconsideration" so it can be amended, if that is possible.

Despite the outcome ultimately resting on the content of the Bill and how it might interact with UK law, Mr Yousaf suggested that was irrelevant to the legal challenge.

In previously unreported remarks made on the Edinburgh Fringe last month, he said the context was senior Tories being anti-devolution and wanting to undermine Holyrood.

After interviewer Iain Dale argued the veto was justified as the Bill could have an effect in the rest of the UK, the FM told the broadcaster: “There’s no argument from the Westminster Government - even from the Westminster government -  that GRR is devolved. It’s a devolved competency.

"You can’t have self-government that is 'Only when we tell you you’re allowed to govern for yourself'. That’s not true self-government.

“That’s the point of the principle. It’s GRR, which we all know there will be a range of opinion [about]. Half this auditorium may well support GRR, half will disagree with it.

“It’s not actually about the legislation itself. It’s the principle. 

“If the parliament passes by a majority a piece of legislation, why can another government come in and veto that legislation? Even if it was legislation I completely disagreed with, I would absolutely fight for the principle.”

He went on: “I believe what's happened is you've got a UK Government, a Secretary of State for Scotland and a Scotland Office who are just looking at any wedge issue they possibly can and deciding, We'll use our powers to overrule.” 

Deputy Scottish Tory leader Meghan Gallacher said: “Humza Yousaf may claim that the challenge to Section 35 is about the principle, but it is clear that this unprecedented action was forced upon the UK Government because the scope of the Bill reaches beyond the competency of the Scottish Parliament.

“Most Scots oppose the GRR Bill and they don’t want to see public money squandered on a needless legal battle to defend it, when ministers should be going back to the drawing board to fix this flawed policy.

“It is yet another example of the SNP manufacturing pointless grievances for which the taxpayers foot the bill, while neglecting Scots’ real priorities.”