With just two days to go before the launch of the cross-party campaign to convince Scots to stay with the Union, the former chancellor says he is convinced Alex Salmond wants a second question on the referendum ballot paper because he is fearful of defeat on independence. Mr Darling says poll after poll shows voters are heavily opposed to it.

"It's obvious to me, to put it bluntly, he wants two questions because he is frightened of one," he says.

Denouncing the idea of a devo-max question on the ballot paper as a "cheap gimmick", Mr Darling argues: "I find it difficult to see how on earth he can put forward with any credibility the proposition for more devolution when he doesn't believe in it."

The No campaign leader denounces the recent SNP attacks against his time at the Treasury, noting: "It's a sign of the Nationalists' weakness that, whenever they are confronted with the merits of what they are proposing, they prefer to attack the individual rather than deal with the question in hand."

However, it is his remarks about the SNP's proposal for an independent Scotland forming part of a currency union with England that have the sharpest edge.

Mr Darling says: "The lunacy of their argument is that to make a common currency work you would have to have all sorts of eurozone-style rules requiring your budget to be approved by the other country, and you end up, as you have seen in Europe, with a common currency that takes you all the way round to economic and then political union. In other words, you go through all the trauma of breaking up the United Kingdom to end up back with it, which is absolutely barmy."