MSPs unanimously approved the bills on children's rights and local government last month, but they will now not receive the royal assent needed to become law until after the legal challenge.

UK ministers are concerned the bills could force them to act, and that the legislation is outside the extent of Holyrood's devolved powers.

Scotland's First Minister Nicola Sturgeon branded the move "jaw-dropping" as she questioned why the Tory government is challenging laws in the Supreme Court that were passed unanimously.

"For what? To protect their ability to legislate/act in ways that breach children's rights in Scotland. Politically catastrophic, but also morally repugnant," the SNP leader said in a tweet on Monday.

The legislation incorporates the United Nations Convention on the Rights of the Child and the European Charter of Local Self-Government, both of which are currently outside of Scottish law.

The UK government has used the powers it is afforded under the Scotland Act 1998, giving it the right to challenge bills passed in the devolved Scottish parliament during a four-week window.

The move is almost unprecedented, with only one other bill having been subject to a legal challenge in more than two decades of devolution.

That bill was a piece of legislation passed in 2018 about keeping EU laws after Brexit, which the court found to be generally sound, before it was superseded by other Westminster legislation.

UK PM Boris Johnson has also sparred with Sturgeon recently over the possibility of Scotland holding another referendum on its independence from the rest of the UK.

Scottish independence is a key part of Sturgeon's reelection campaign ahead of the May elections, but has been repeatedly resisted by Johnson. The PM only last month called devolution an SNP "obsession" and "the last thing" Scotland needs.