THOUSANDS of laws affecting devolved issues could be unilaterally changed or scrapped by Westminster as a consequence of Brexit, a leading legal expert has warned.

In a report to Holyrood’s European Committee, Professor Alan Page said the UK government would rely heavily on obscure secondary legislation to unpick EU laws after Brexit.

However, unlike reforms affecting Scotland based on UK Acts of Parliament, such changes would not require the consent - or even the knowledge - of MSPs.

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The upshot could be Westminster rewriting laws in Scotland by the backdoor, he suggested, calling it “a significant potential gap” in law making in devolved areas.

Prof Page, Professor of Public Law at the University of Dundee, and a former adviser to the UK and Scottish parliaments, said Holyrood stood to gain many new powers after Brexit, notably over agriculture, fisheries and the environment, once EU law no longer had primacy.

Control of public procurement, animal health and welfare, food labelling, water standards, GMOs, climate change and and aspects of energy would revert to Edinburgh.

Brexit would also end the need for the Scottish Government to abide by EU rules on state aid, allowing it to prop up businesses with grants, or comply with EU competition law, meaning its plan for minimum unit pricing for alcohol could not be stymied in the courts.

Prof Page's report, The implications of EU withdrawal for the devolution settlement, was published a day after Downing Street signalled Holyrood would be denied any kind of meaningful vote on the Great Repeal Bill paving the way for Brexit.

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The Bill would automatically embed 45 years of EU law into UK statute, providing continuity after Brexit.

Westminster and Holyrood would then be free to keep or repeal these laws as they saw fit.

But Prof Page said London would also have the power to unpick laws affecting Scotland without consulting Edinburgh, through the use of subordinate, or secondary, legislation.

He wrote: “At the moment there is no requirement for the Scottish Parliament’s consent to UK subordinate legislation transposing EU obligations in the devolved areas; nor is the Parliament routinely informed about such legislation.

“The situation could thus arise in which the UK legislated extensively in areas devolved to Scotland without seeking the consent of the Scottish Parliament as there would be no requirement of its consent in relation to subordinate legislation altering the effects of EU law in the devolved areas.

“In my view, this represents a significant potential gap in the framework of EU law making in the devolved areas, which the Scottish Parliament should be alert to the need to close should UK Ministers be given the power to revise EU law in the devolved areas.”

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Committee convener Joan McAlpine said: “It is clear from this paper that, even with many EU powers likely to be reserved to Westminster, there will be a significant impact for Scotland.”

Prof Alan Page added: “The process of the UK withdrawing from the EU, after negotiations have taken place, will have a crucial impact on the Scottish Parliament and its powers.

“The Scottish Government will have to decide how it will legislate on powers returned from Europe and a lot of detailed scrutiny will be required by Holyrood’s committees.”

A Scottish Government spokeswoman said: “The possible impact signalled by this report on devolved interests only reinforces the need to deliver the Prime Minister’s commitment for full engagement and involvement of the Scottish Government before Article 50 is triggered.”