BREXIT is straining the convention that underpins law-making by Edinburgh and London “almost to destruction”, one of the country’s leading legal academics has warned.

Aileen McHarg, professor of public law at Strathclyde University, said the UK government had failed to justify its unprecedented breach of the Sewel convention.

Writing in today’s Herald, she says supporters of devolution “should be very concerned about what happens next” in light of the furious row over the EU Withdrawal Bill.

The Sewel Convention, named after the Labour peer who piloted the 1998 Scotland Act through the Lords, relates to legislation in devolved areas.

It says that, although Westminster is sovereign and can in theory make any law it likes, it will “not normally” act in devolved areas without Holyrood’s consent.

When the UK Government last week pressed ahead with the EU Withdrawal Bill without Holyrood’s agreement it marked a first for devolution.

The move prompted SNP MPs to walk out the Commons complaining of power grab that will see some devolved powers repatriated from Brussels to Westminster.

Prof McHarg said although it was possible to depart from the Sewel Convention, UK ministers had not given any serious constitutional grounds for doing so, merely citing Brexit as an abnormal situation.

She said: “To make an exception to a rule, what is required is an explanation of why its underlying rationale either does not apply, or is overridden by some competing principle.

“The UK Government seems simply to be saying that so long as they tried to reach agreement with the Scottish Government, the Convention is satisfied.

“But this is a radical re-reading of the Sewel Convention which would seriously undermine the protection it offers for devolved autonomy.”

Lord Sewel, who quit Westminster in 2015 after a tabloid expose into his private life, told BBC Scotland yesterday he agreed with the UK Government’s action.

He suggested the SNP was mischief-making and denied there was a constitutional crisis.

The size and scale of Brexit meant the UK government had to be allowed to “act on its own initiative” without the Scottish Parliament’s backing, he said.

He added: “It is not unknown for political parties to seek political advantage over these sort of issues, and there's a fair bit of that going on. It's not a constitutional crisis.”

Tory MSP Adam Tomkins, who is a law professor at Glasgow University, said Prof McHarg was not a neutral commentator, as she co-convened “Lawyers for Yes” in 2014.

He said: “Contrary to her one-sided analysis, the Sewel convention has not been breached by either the UK Government or by the Westminster Parliament.

“Sewel provides that consent will normally be required before Westminster legislates on matters relating to devolution. In this case, no less an authority than [SNP Brexit minister] Mike Russell himself has said ‘these are not normal times’.

“The SNP and its supporters cannot have it both ways.”