The intervention adds to pressure on the Prime Minister over the constitutional question and who should ultimately have the authority to call another independence vote.

The Greens want that power to rest with the Scottish Parliament rather than requiring agreement from the UK Government.

Their position is that if a majority of elected MSPs vote in favour of holding a referendum, that mandate should be respected.

Mackay said:

“It is a democratic outrage that the First Minister is having to write to the Prime Minister in the first place.

“The right to hold a democratic referendum should rest with the people of Scotland.

“If a majority of elected MSPs in the Scottish Parliament vote to hold a referendum then that democratic mandate should be respected.

“We have already endured far too many years of obstruction, hurdles and political games from UK governments terrified of allowing the people of Scotland to decide their own future.

“It has gone on for far too long.

“The Scottish Parliament should have the power to trigger that referendum, rather than a UK Government hundreds of miles away.

“I hope that the Prime Minister responds in good faith and that he lives up to the promises he has made to decentralise power and end the tired and broken ‘Westminster knows best’ approach of his predecessors.”