The leading Scottish historian Sir Tom Devine, who is backing a Yes vote in the referendum, has said if the vote is No the SNP may "come to regret" rejecting a third option on the ballot paper.

Writing in the new London Review of Books, Professor Devine said that, with polls still showing the No voters ahead of Yes, a different strategy could have been pursued.

The historian writes that the initial preference of the Scottish Government was to have three questions, with "devo-max" added to the other two.

He writes: "The UK government was emphatically against that. Two questions it would have to be.

"Strong rumour suggests that Alex Salmond would have preferred to go for devo-max - which would undoubtedly have been accepted by a massive majority of the Scottish electorate - and then, after bedding down the new powers, to go to the country again in a few years' time on the final step of full sovereignty for Scotland.

"But the possibility of achieving the holy grail of Scottish independence in a single vote was just too tempting."

He concludes: "With the 'No' campaign still in the ascendancy as the day of reckoning looms, the pro-independence camp may yet come to regret its rejection of gradualism."