Two Bills passed unanimously by Holyrood could give Scottish courts “unparalleled” powers to strike down legislation from Westminster, the UK Supreme Court has heard.

Lawyers acting for the UK Government said the legislation risked Acts of Parliament being amended or repealed because of clashes with Scots law.

They argued this would be a “subversion” of devolution, under which Westminster remains sovereign, with both Bills having “manifold and obvious competence problems”.

They were passed by MSPs in March and deal with local government and children’s rights, but they were challenged by the UK’s law officers for placing obligations on UK ministers.

The UK Government has said their concerns “are not about the substance of the legislation”, but whether the Scottish government has the legal ability to pass the Bills.

Alister Jack, the Scottish Secretary, wrote to the SNP government before the legislation was passed, pointing out the potential problems and suggesting amendments.

But John Swinney, the Deputy First Minister, dismissed the approach as “menacing” and a “power grab”. 

After the legal challenge was announced during the Holyrood election campaign, Nicola Sturgeon, the First Minister, said the move was “politically catastrophic and morally repugnant”.

Bills under the spotlight

A panel of five judges on Monday heard arguments from the UK Government about why they thought the legislation falls outside Holyrood’s competence.

The Scottish and Welsh governments will make representations on Tuesday on the second and final day of the hearing. A judgement will be issued at a later date.

In written arguments, Sir James Eadie QC, acting for the UK Attorney General and the Advocate General for Scotland, said: “Both Bills, in slightly different ways, purport to bestow upon the Scottish courts extensive and, in part, unparalleled powers to interpret and to scrutinise the legality of primary legislation passed by the sovereign UK Parliament at Westminster.”

The Bills aim to incorporate international treaties signed by the UK into Scots law, with the first the United Nations Convention on the Rights of the Child (UNCRC) (Incorporation) (Scotland) Bill.

This aims to set a legal requirement for public authorities to comply with certain standards on children’s rights and gives Scottish courts the power to decide if legislation is compatible with them.

The second is the European Charter of Local Self-Government (Incorporation) (Scotland) Bill. 

However, Sir James said: “There are, we submit, manifold and obvious competence problems. The relevant Bills subject Acts of the UK Parliament, both past and future, to what is in effect characterised as a superior body of law, the UNCRC and the European Charter.”

In written arguments, James Mure QC, representing the Lord Advocate for Scotland, said that none of the provisions modify the law on the relationship between the powers of Westminster and Holyrood “on a proper reading”.