BREXIT will have a “pretty catastrophic” impact on the UK unless the free movement of people continues for several years, according to Nicola Sturgeon’s top EU adviser.

Professor Anton Muscatelli, the principal of Glasgow University and chair of the First Minister’s standing council on Europe, also warned a quick trade deal with the EU was “complete fantasy”.

Addressing a business breakfast in Glasgow, Prof Muscatelli said he and other experts had a “duty” to point out the flaws in a hard Brexit that would take the UK out the EU single market.

He said: “We are actually heading, in my view, for a pretty catastrophic outcome unless we have a relatively soft Brexit.

"Some sort of single market solution, at the very least as a transition, is frankly the best thing we can even hope for. Because you're not going to get a negotiated free trade agreement in two years' time, it's a complete fantasy.

"The best we can hope for is some transitory deal which will mean freedom of movement in the short run, which will then give you a platform for a longer discussion, whether it is along the Swiss lines or yet another model that is still to be invented."

Although not in the EU, Switzerland has access to the single market as a member of the European Free Trade Association (EFTA).

He suggested there should be a “differentiated approach by nation, region and sector” to immigration, but admitted the UK government had not been sympathetic to it so far.

However a former Labour minister said Brexit would force politicians to “put their money where their mouth is” and train people properly for jobs rather than rely on EU migrants.

Tom Harris, the former MP for Glasgow South who led the Leave campaign in Scotland, said the country could no longer rely on simply finding skilled workers from overseas.

He said: “For the first time our politicians are living in an environment where there is at least a possibility that if we don't train up Scottish youngsters to fulfil those roles we may not be able to rely on an unending supply of foreign workers to do those jobs for us.

"It puts a huge amount of pressure on politicians and it's something we should welcome, that for the first time in a generation these politicians are actually going to have to put their money where their mouth is and actually deliver on training of the indigenous population.

"They've been allowed to ignore that for too long because they've always been aware the safety net of EU immigration has been there."

Dr Peter Bennie, chair of the British Medical Association in Scotland, said Scotland could build on a very limited existing ability to have different migration policies in different parts of the UK.

He said: “There's a UK shortage occupation list, which includes a number of medical specialities where we simply don't have enough home graduates to fill those places at present. There's a separate and additional Scottish shortage occupation list, which includes additional specialities. It seems to me that works very smoothly already for non-EU, non-EEA citizens.

"Therefore we've got at least the bones of that kind of structure, and I think it could work."