THE SNP is to rely on the Act of Union, the law it has fought more than 80 years to repeal, in its Supreme Court intervention over Brexit.

Nicola Sturgeon’s most senior law officer will cite the 1707 Act to argue it would be "unlawful" for Theresa May to trigger Article 50 and leave the EU without a new law at Westminster.

In a 58-page submission to the UK Supreme Court published last night, the Lord Advocate James Wolffe QC argued Holyrood should also have a vote, though not a veto, on Article 50.

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It said: “It is a matter of constitutional principle that laws cannot be amended or repealed by an exercise of the royal prerogative alone. This principle is reflected in the Claim of Right Act 16989 and in Article XVIII of the Acts of Union of 1706 and 1707.”

Mr Wolffe also said EU withdrawal triggered all three grounds for MSPs to have their say through a legislative consent motion (LCM), as it would alter the competence of both Scottish Ministers and the Scottish Parliament, and affect devolved policy areas.

“The effects of withdrawal from the EU on devolved matters are such as to engage the legislative consent convention under which the UK parliament will not normally legislate with regard to devolved matters without the consent of the Scottish Parliament.

"The constitutional requirements.... , according to which any decision to withdraw from the EU must be taken, accordingly include both the legal requirement for an Act of the UK Parliament, and the legislative consent convention."

The High Court ruled earlier this month that the UK government cannot exercise the royal prerogative to trigger Article 50 and remove rights accumulated since the 1972 European Communities Act; only parliament can undo acts of parliament, it said.

The UK government is now appealing that decision to the UK Supreme Court, where 11 justices are due to start a week-long hearing on December 5, and give their view in January.

The Scottish and Welsh governments have been granted the right to intervene in the appeal, allowing the Lord Advocate to present his case in favour of an LCM for Holyrood.

Written with the help of advocate Duncan Hamilton, a former SNP MSP, the argument even quotes the preamble to the Claim of Right Act on the “wicked and evill Counsellers” who advised King James to turn “a legall limited monarchy to ane Arbitrary Despotick power”, in a section on why only parliament was able to annul laws.

It also quoted a paper by the Tory MSP Adam Tomkins, a law professor at Glasgow University.

The 62-38 rejection of Brexit north of the border means Holyrood would be likely to reject triggering of Article 50 if granted an LCM.

This would not be a veto, as Westminster remains sovereign, and it is only a convention that it stays out of devolved law-making.

However if Westminster did ignore Holyrood, it would mean a constitutional crisis, but not as big a crisis as Brexit collapsing.