THE pro-UK campaign has called on the Scottish Government to make clear what its Plan B for a currency in an independent Scotland would be.

The move comes after Chancellor George Osborne said the UK would be unlikely to agree to a currency union.

The SNP argues that a sterling zone between an independent Scotland and the UK would be the best, most common sense option should Scots vote yes in next year's referendum.

On Tuesday Mr Osborne made clear the economic risks to the UK could be too great for it to agree to such a currency union.

In response, Alex Salmond, the First Minister, dismissed this as scare-mongering and "political sabre-rattling".

However, the Better Together campaign insisted Nationalist credibility on the currency was being called more and more into question and that the "voices for a Plan B from the Scottish Government are getting louder".

It pointed to remarks by Professor John Kay, a former economic adviser to the Scottish Government, who said that if he were representing it in negotiations with Whitehall following a yes vote, he would "try to secure a monetary union with England and expect to fail".

He argued an independent Scotland would have bargaining power only if it held open the option of a separate currency and that was the only option campaigners for a Yes vote could commit to deliver.

"The question that remains is what to call the new money," he said. "Perhaps the Scots crown or the pound Scots. But I prefer the resonance of the coin first minted by the father of Mary Queen of Scots – the bawbee."

At Westminster, the currency issue was also raised during Scottish Questions when Angus Robertson for the SNP pointed out how the IMF had downgraded its forecast for UK growth and had questioned the Coalition's austerity programme.

"Why," he asked, "should anyone believe a word the Chancellor or the Chief Secretary to the Treasury say on the Budget, on currency, or anything else for that matter?"

Michael Moore, the Scottish Secretary, replied: "Nobody will be listening to the Honourable Gentleman and his party when it comes to the currency."