RUTH Davidson had her commitment to devolution questioned yesterday, after a new biography of the Scottish Tory boss claimed she was “f***ing furious” about The Vow.

The promise to deliver new powers to the Scottish Parliament if Scots backed a no vote in the 2014 referendum on independence was supported by David Cameron, Nick Clegg, and Ed Miliband, and was printed on the front page of a national newspaper on September 16, just two days before the voters went to the polls.

Andrew Liddle, the former journalist-turned Labour press officer, who has written the new biography, said Davidson thought that the pledge was unnecessary and would hand the initiative to the SNP after the vote.

In his book, Ruth Davidson and the Resurgence of the Scottish Tories, Liddle writes: “Ruth was — in the words of one senior party insider — ‘f***ing furious’ with Cameron over the Vow. While she had been consulted over the pledge, she was strongly opposed.

“The Scottish Tory leader — rightly — argued that the Vow would play right into the Nationalist narrative. SNP leaders would be able to suggest that they did not lose on the question of independence, but rather the vote was one about more powers.”

Liddle adds: “One senior insider, who was with Ruth when the Vow was being discussed, said: ‘It was the only time she got angry during the campaign.

“She felt that this would allow the SNP narrative to get going again ... There were some interesting scenes in the Better Together office.’ ”

The Vow has become a totemic issue for many on the pro-independence side, with some firmly believing it was the promise of new powers for Holyrood that helped No grab their 55% to 45% victory.

However, a survey of more than 4500 people in 2015 by Edinburgh and Essex universities revealed that just 3.4% of No voters said the offer of more powers was the main motivation for their decision.

A senior Scottish Tory source confirmed the accuracy of the claims made in the book to a UK newspaper.

He said: “This is all now ancient history. But given what happened after the referendum, history might conclude Ruth had a point.”

Scottish Labour leader Richard Leonard called it “a shocking revelation” which “blows apart Ruth Davidson’s claims to be a different type of Tory.”

He added: “However, all we have to do is look at how the Scottish Tories have behaved over the EU Withdrawal Bill to see what they think about more powers for Scotland.

“It just goes to show you can’t trust the Tories to defend devolution – only Labour stands for a strong Scottish Parliament inside the UK.”

The SNP’s Ash Denham, who sits on Holyrood’s Constitution Committee, said it was proof Davidson’s commitment to devolution was “paper-thin”.

Denham commented: “Every statement Ruth Davidson makes on the powers of the Scottish Parliament is entirely about short-term political calculation and nothing to do with any sincerely held beliefs – it’s almost as if there are several Ruth Davidsons roaming around out there, each acting under orders from Westminster to tell people in their immediate vicinity what they think they want to hear.”