One of Nicola Sturgeon’s handpicked Brexit advisers has poured cold water on her proposals to keep Scotland in the EU single market even if the UK leaves, only hours before she unveils them on Tuesday morning.

Charles Grant, who sits on the First Minister’s Standing Council on Europe, said it was “extremely difficult” to see how her plans were legally, politically or technically feasible.

In an interview with the Telegraph, he said Scotland staying in the single market without the rest of the UK would require the complete devolution of business regulation and that “clearly isn’t going to be on the cards in the foreseeable future.”

He said proposals for Scotland to stay in the EU’s customs union, a more limited type of free trade area than the single market, without the rest of the UK would also mean customs checks at the border with England.

Among the other flaws he listed were Spain blocking any special deal and Theresa May being unwilling to sign up to a package of powers that would give Scotland “something like dominion status.”

Scottish Government sources said a report on Scotland’s EU options being unveiled by Ms Sturgeon on Tuesday morning represents the stance of her SNP administration and not the group of experts she assembled, among whom they said there is a “diversity” of views.