JOHN Swinney has repeatedly failed to identify the currency of an independent Scotland.

In a bruising interview on BBC Radio Scotland, the former Finance Secretary also refused to discuss the SNP Growth Commission which is updating the party’s independence offer.

The Commission's chair, former MSP and RBS economist Andrew Wilson, recently admitted oil revenues were wrongly described as a “bonus” in the 2014 referendum, instead of a “basis” of the economy, supplying around 12 per cent of national income.

The Commission is also examining whether to drop the past policy of sharing the pound and switching to a separate Scottish currency.

Mr Swinney was asked six times which currency Scotland would use if Nicola Sturgeon secured and won second referendum between autumn 2018 and spring 2019.

He said: “Well, if the referendum is taking place within that period, and that is what we want to negotiate with the United Kingdom government, that is where we will set out the arguments and the basis and the alternative prospectus to Scotland being forced to accept a hard Brexit from the European Union courtesy of the United Kingdom government.”

Asked what the Commission had recommended, Mr Swinney said: “Obviously material, documents, policy positions will be set out well in advance of that period.”

Pressed again, the Deputy First Minister said: “We will present the arguments, as we did comprehensively and fully in advance of the referendum in 2014.”

Asked if the Commission had identified a single currency option, he said: “I am not going to discuss the details of the Growth Commission when that will be published in due course in advance of the referendum on independence.”

SNP MP Joanna Cherry QC also appeared to struggle when she was asked to name the currency by a member of the audience on BBC Question Time on Thursday night.

After initially stumbling over her words, she said the SNP would not tell people until much closer to an independence referendum, saying: “Watch this space.”

Labour claimed the SNP’s economic case for independence was unravelling daily.

Economy spokeswoman Jackie Baillie said: "The fact that John Swinney can't even answer a basic question about what currency a separate Scotland would use says it all.

"The currency we use really matters. It's about how much money we have to spend on schools and hospitals, what money wages and pensions will be paid in, and will determine how families pay the bills. The Nationalists want to drag Scotland out of the UK but haven't even worked out the basics like currency.”

In a separate interview on BBC Radio 4, Mr Swinney also refused to rule out a third referendum if a second vote resulted in defeat for the Independence side.