SIR Menzies Campbell called for the three Unionist parties to sign up to a guarantee of greater devolution in the event of a No vote in the referendum.

The ex-LibDem leader urged Labour and the Tories to join his party for a "common way forward" on more powers for Holyrood.

The issue was so important it demanded the parties work together "in the national interest", he said.

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In a question-and-answer session at the Scottish LibDem conference, Campbell said the way the aftermath of the poll would be "a point of very considerable significance".

He called for "heads of agreement" to be set out by the Tories, Labour and the LibDems before September.

"They don't have to give the detail, we know Labour's position is different to ours, we know the Tories' is different to ours in detail, but we know there are principles which all three parties say they are bound to, and principles which would give rise to a common statement of intent," he said.

Last week, Labour published their proposals for more powers; the Tories will do likewise in May.

Campbell called on the ­leadership of all the political parties to set out "a common way forward".