BORIS Johnson has been warned by a law professor that his plans for a UK internal market could pose a threat to devolution.

The UK Government has drawn up a white paper to act as a blueprint for trade after the Brexit transition period ends on December 31.

But professor of public law and human rights at Durham University, Aileen McHarg, has stressed that the proposals do not make it clear if devolved administrations such as the Scottish Government would be able to override new constraints on "market access".

The Scottish Government has announced it will not recommend legislative consent for the UK Government's Internal Market Bill.

READ MORE: SNP vows to thwart UK internal trade bill "at every stage"

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

UK ministers have said it will result in 111 powers being transferred to Holyrood from Brussels - although Mr Russell insisted that claim is a "lie".

But Michael Gove, Chancellor of the Duchy of Lancaster, claimed Mr Russell had “misrepresented” the UK Government’s plans.

Speaking on BBC Radio Scotland's Good Morning Scotland, Professor McHarg said the devolved parliaments and 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."

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

READ MORE: Gove accuses Russell of 'misrepresenting' Westminster trade plan

She 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."

She said that “potentially it could” threaten devolution.

She added: "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.

Prof McHarg added: "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."

The white paper is open to consultation for four weeks and the legislation is intended to be in place by the end of the year, a period Prof McHarg said is "very short" to allow negotiations to take place.