SCOTLAND’S Constitution Secretary has warned Boris Johnson’s Conservative Government is plotting to use Brexit to carry out the “biggest power grab yet” on Holyrood.

Michael Russell has written to Cabinet Office Minister Michael Gove urging the scrapping of proposals for a “UK internal market” following its EU exit, which he says would undermine devolution and the democratic choices of the Scottish Parliament.

The introduction of a “market impact test” for legislation could jeopardise the existence of flagship Scottish policies, such as minimum unit pricing for alcohol and free university tuition fees, the letter states.

And a requirement to have the same regulatory standards across the UK could lead to Scotland having to automatically accept chlorinated chicken imports if they were permitted in England, it is feared.

Russell told the Sunday National: “This is a red line and we are simply not doing this.

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“The agenda here is clear – there is a hostile agenda to devolution and the action being taken under the cloak of Brexit is to weaken devolution.”

The UK has previously not had an internal market defined in law, as it followed the rules of the European Union internal market law – designed to ensure free movement of goods, services, capital and workers.

Differences across the nations were allowed under EU law as long as they met certain conditions, such as public policy objectives.

In his letter to Gove, Russell says it is understood the UK Government now plans to introduce legislation before 2021 which would enshrine the “UK internal market” in law.

But he added: “The information to hand leads the Scottish Government to believe that the purpose of such plans is not economic but is instead purely political.”

He said of particular concern was a proposal to establish an unelected, external body that would judge whether any new bill in the Scottish Parliament met a “market impact test”.

“The Scottish Parliament itself is the appropriate place for any concerns about the market impact of legislation to be addressed – not an unelected panel appointed by UK ministers,” the letter says.

“Our initial assessment is that if such a market impact ‘test’ had been available in previous years, there would have been a significant risk that important and successful policies, which have attracted widespread public support, such as the imposition of minimum unit pricing, tuition fees policy and the ban on smoking in public places, would have been among the devolved policy measures that could well have been caught up in these new arrangements. Indeed they could still be challenged, depending on the powers and functions of the proposed body.”

A second key issue is a provision for a new “mutual recognition regime” that could require regulatory standards in one part of the UK to be automatically accepted in others – regardless of whether those standards are lower than those the Scottish Parliament might find acceptable.

Russell said this would mean, for instance, that if chlorinated chicken was allowed to be imported to England, Scotland would have no option but to permit it.

In the letter he stated: “This proposal is not only unacceptable: it ignores the reality of devolution.

“These have shown that the market can successfully operate across the UK with variations in standards, in ways that allow for positive differences in ambition to be pursued by the four nations.

“It would be extremely damaging if these freedoms were lost, or we could no longer make choices.

“Your approach would work against the interests of producers and consumers, and ignore the need to reflect natural variations in our geographies, sectors and communities which is a cornerstone of devolution.”

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Johnson’s Government is expected to publish a preliminary Green Paper report outlining the proposals on the UK internal market by the end of this month.

Russell said the Scottish Government had not taken part in talks with the UK Government on the issue as it did not accept there was a need for them.

He added: “The Tories know they can’t win an election to the Scottish Parliament so have come up with a scheme to undermine it instead.

“They’re plotting to introduce a new law which would effectively hand power to Tory ministers in devolved policy areas, under the disguise of protecting the so-called UK internal market after Brexit.

“Under their plans, if Westminster adopts lower standards in devolved areas, Scotland could be forced to accept them, regardless of the view of our Parliament.

“And they are considering setting up an unelected oversight body to set a new ‘internal market’ test for any new Holyrood legislation.

“This scheme amounts to using Brexit as a cover to mount the biggest power grab on the Scottish Parliament yet and we will do all we can to stop it from happening.”

A spokesman for Michael Gove yesterday accused the SNP of “confecting yet another political row” to pursue their “separatist agenda”.