THE UK Government's plans for an "internal market" after Brexit could threaten devolution, an academic has warned.

Professor of public law and human rights at the University of Durham Aileen McHarg said it is not clear if devolved governments would be able to override new constraints on "market access".

She was speaking after the Scottish Government announced it will not recommend legislative consent for the UK Government's Internal Market Bill.

Constitution Secretary Michael Russell has branded the measures the "biggest threat to devolution since the Scottish Parliament was reconvened in 1999", as he vowed ministers at Holyrood will "actively oppose the UK Government's proposals at every opportunity".

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The UK Government published its white paper yesterday, setting out plans for trade after the Brexit transition period ends.

UK ministers' claims that it will result in more than 100 powers being transferred to Holyrood from Brussels were branded "lies" by Russell yesterday. 

Speaking on BBC Radio Scotland's Good Morning Scotland programme today, McHarg explained that the devolved assemblies are currently required to legislate in accordance with EU law but that obligation will fall away at the end of the year.

She said: "What the white paper is proposing is a new obligation to legislate in accordance with a new market access commitment.

"This is a new constraint which will replace the current constraint imposed by EU law.

"The difficulty is that the new constraint is unlikely to be the same. It looks like, though the details are not entirely clear, it will be broader in its application."

She gave the example of the Scottish Government's minimum alcohol pricing law, which faced a legal challenge on EU competition grounds which was ultimately unsuccessful.

McHarg said: "We just don't know yet whether there will be the same possibility under the new proposals for the devolved parliaments to override these new cross-cutting internal market constraints."

Asked if this could threaten devolution, she said: "Potentially it could.

"It also looks like it's going to cover a wider range of policy areas."

Proposed rules around building regulations suggest the new internal market constraints could be wider than EU law, she said.

McHarg continued: "There's a concern because one of the things that's going to be required is something called mutual recognition.

"If goods or services are authorised, or are lawful in one part of the UK, then they can be lawfully sold or provided in any other part of the UK.

"The difficulty is because England is so large and so dominant.

"That means that anything produced in England and according to the English standards can be supplied in Scotland, Wales, Northern Ireland, and because the UK Parliament legislates for England, because it's not in practice bound by any ultimate guarantees, then English standards in practice are likely to prevail."

Legislation is planned to be in place by the end of the year, a period Prof McHarg said is "very short" to allow negotiations to take place.