NICOLA Sturgeon’s plan for Scotland to stay in both the European single market and customs union after Brexit is “almost impossible”, a leading expert has warned.

The First Minister’s detailed proposals for a soft exit for Scotland were already facing huge political challenges, not least from the British Government.

Dr Kirsty Hughes, of the think-tank Friends of Europe, has spotted what she believes may be an insurmountable bureaucratic hurdle.

Read more: Theresa May makes clear UK's departure from EU will not be 'half in, half out'

Ms Sturgeon and her experts propose that the country could join the European Free Trade Association (EFTA), the grouping of Norway, Iceland, Switzerland and Liechtenstein, which are part of the single market but not the European Union.

This means they abide by single market rules, including free movement of people, but have customs formalities, which may not be barriers or roadside posts, for any trade crossing borders.

But Ms Sturgeon and her team also wanted Scotland to be in the EU’s customs union, allowing customs-free trade on industrial goods. It includes Turkey, which is not in the EU or in the EFTA.

Dr Hughes quoted an anonymous Swiss official as saying: “The EFTA States are not and cannot be part of the EU’s customs union.”

Read more: Theresa May makes clear UK's departure from EU will not be 'half in, half out'

She added: “The Scottish Government’s first option – of the UK staying in the EU single market and customs union – appears impossible. And while the UK looks clearly set to leave the EU’s single market, its position on the customs union is not yet clear.

“If the UK stayed in the EU customs union, Scotland could not be in both EFTA and the EU customs union raising a potential problem for pan-UK trade – unless it was independent and in the EU (and so in the customs union).”

Dr Hughes said that nuances over EFTA and the customs union had not been noticed by experts and officials in London, Brussels and Edinburgh.

One Norwegian politician has already signalled that a non- independent Scotland would not be welcome in EFTA, but the Faroes is an example of a non-sovereign state on the European stage.

Read more: Theresa May makes clear UK's departure from EU will not be 'half in, half out'

A government spokeswoman said: “Brexit is unprecedented and so calls for unprecedented solutions.The detailed proposals we published last month are designed to keep Scotland in the European single market – which is around eight times bigger than the UK’s alone – even if the rest of the UK leaves. That is absolutely essential for Scottish jobs, investment and long-term economic wellbeing.”