UNIONIST politicians must keep their promises of more devolution if Scotland votes No to independence, historian Sir Tom Devine said.

Some people have said core nationalists are heading for "a collective nervous breakdown" if Scotland votes No, Sir Tom told an audience in Edinburgh.

He said it may not be that bad but that "it will certainly be much worse than 1979" when Scotland was promised more powers if it voted No to devolution and Westminster failed to deliver.

Sir Tom became the first Scottish historian to receive a knighthood this week in the Queen's Birthday Honours, and is retiring from his role as director of the Scottish Centre for Diaspora Studies at the University of Edinburgh this summer.

He held a public conversation with former prime minister Gordon Brown, a prominent unionist campaigner, to mark his retirement.

Sir Tom said he intends to cast a vote in the referendum, but refused to reveal his constitutional preference.

He said a Yes vote would be followed by "a period of very exciting activity".

But, he said, No would be "much more difficult" to manage because of the collective disappointment of core nationalists.

"Even more important than a Yes vote will be the discussions that take place afterwards," Sir Tom said.

"A No vote is much more difficult because, there is no doubt about it, people who support Yes, that core of 30 per cent who through thick and thin has gone for it, if there is a No vote then they are going to be terribly, terribly disappointed.

"Some people have said 'a collective nervous breakdown'.

"It might not be that but it will certainly be much worse than 1979."