When Mr Salmond attempted to repeat his interpretation of Mr Darling’s comments, he was booed by the audience for avoiding the question. The Labour MP repeated that he made clear in the interview that a currency union without political union is “stupidity on stilts”.

Mr Darling said the pound was not the currency of any of the four home nations but of the UK as a whole and separating would mean walking away from sterling.

The First Minister continued to insist it was as much Scotland’s currency but the former Chancellor said he could not force the people of England, Wales and Northern Ireland to accept a deal to share the pound.

Ratcheting up the pressure on Mr Salmond, Mr Darling attempted to force him to name his Plan B currency using a process of elimination.

The First Minister said he did not want to join the euro or adopt a new currency, with the former Chancellor saying the only remaining choice was using the pound in the same way countries like Panama use the dollar.

But Mr Darling said this would be disastrous for Scotland’s financial services industry, which could no longer rely on the Bank of England as lender of last resort.

Mr Salmond said it was in the interest of the remainder of the UK to enter a currency union for trade reasons but his opponent pointed out it would mean taxpayers south of the Border taking responsibility for bailing out banks in another country.

Mr Salmond used his cross-examination of Mr Darling to ask whether the pro-UK campaign considered itself to be “Project Fear”. He also repeatedly referred to there being more pandas in Scotland than Tory MPs.