AN independence referendum called without Westminster consent could be both illegal and constitutional, a leading politics expert claims.

Aberdeen University’s Professor Michael Keating says a “paradox” means a ballot called by Edinburgh alone could be legally illegitimate and constitutionally sound at the same time due to the way UK rules are drawn up.

And he says these rules must be changed “if the UK is to survive as a union of nations rather than a decentralised unitary state”.

Keating – who will next month release his new book State and Nation in the United Kingdom: The Fractured Union – makes the claim in a new piece for the Scottish Centre on European Relations (SCER) think tank.

The intervention comes one month after a court dismissed a landmark challenge over whether or not the Scottish Parliament needs Westminster approval to proceed with a fresh vote on the constitution.

Known as the “Keatings case”, the crowdfunded case was taken before the Court of Session by Martin Keatings of the Forward As One group.

Lawyers acting for the UK Government argued that the petitioner lacked the “standing” needed to bring the matter to court because he is not a member of the Scottish Parliament.

In her ruling, judge Lady Carmichael dismissed the case as “hypothetical, academic and premature” but did not make a statement on the competency of the action.

There is intense interest around the issue just eight weeks away from the Scottish Parliament election.

Early campaigning on this has revolved around the question. The SNP and the Greens are both in favour of a new referendum following the UK’s withdrawal from the EU, something Scotland did not vote for.

However, the Tories, Labour and the LibDems remain opposed to indyref2.

Writing for SCER, Keating says the “Westminster view based on the absolute sovereignty and supremacy of the UK Parliament” is in conflict with arguments for self-determination, such as that included in the 1988 Claim of Right for Scotland, which was written by the Campaign for a Scottish Assembly and acknowledges “the sovereign right of the Scottish people to determine the form of Government best suited to their needs”.

Keating writes: “If Scotland does have a right to independence, it would be logical that there should be a means to realise it.

“If democracy is a foundational principle of the constitution, that means should be democratic.

“A mandate obtained in an election held under proportional representation, followed by a referendum, would surely meet that test.”

He goes on: “There have been few referendums in the United Kingdom so that it is difficult to find precedents and conventions but there are indications of an emerging practice.

“Referendums on Europe in 1975 and 2016, on devolution in 1997 and on the Alternative Vote in 2011 and on Scottish independence in 2014 were held when a party, having included a referendum in its manifesto, won an election.

“The Northern Ireland Act indicates that a referendum on Irish unification should be held where the Secretary of State judges that there is a majority in favour.”

READ MORE: SNP's indyref2 war chest piles pressure on Unionist parties and Westminster

Arguing that an independence vote could be illegal “but, in another way, constitutional”, he cautions: “It is not likely that the Supreme Court would see it that way, as it has consistently upheld the Westminster doctrine.

“Legal scholars and even judges (when speaking out of court) have argued that the doctrine of Westminster sovereignty is in need of revision in the light of devolution, human rights law and (until recently) EU membership.”

Referring to “the Cherry case” – in which a court challenge brought by Joanna Cherry MP and others established that the Government had acted unlawfully in moving to prorogue Parliament and prevent Brexit scrutiny – he goes on: “The Supreme Court itself insisted on the role of conventions when it overturned Boris Johnson’s attempt to prorogue Parliament; yet it has dismissed the legislative consent provision as merely ‘political’.

“In my new book, I argue that such a revision is necessary if the UK is to survive as a union of nations rather than a decentralised unitary state.”