NEARLY 200,000 viewers tuned in to watch the first live television debate of the referendum campaign, according to STV.

The Scotland Tonight special, which pitched Deputy First Minister Nicola Sturgeon against Scottish Secretary Michael Moore, attracted 190,000 viewers at its peak.

STV also hailed an 850% increase in Twitter traffic, as the show sparked a flurry of online comment.

The hour-long special, the first in a series of live showdowns, focused on the economics of independence, and allowed Ms Sturgeon and Mr Moore to cross-examine each other.

Gordon Macmillan, head of news at STV, said: "The first debate in our series of referendum specials was a great success with extremely encouraging viewing figures and positive feedback.

"The format worked well, particularly with the extensive engagement from our audience via social media, and we look forward to further debates in the coming months."

Ms Sturgeon used the show to claim the SNP's proposed sterling zone – a formal deal with the UK to share the pound – would only place limits on Scotland's debt and borrowing levels, leaving an independent Scottish government full control over tax policy.

However, she refused to reveal whether the SNP had a "Plan B" in case the UK Government refused to agree to a currency union, as reported in later editions of yesterday's Herald.

The UK Government had previously warned a sterling zone was "unlikely" to be agreed because of risks to the rest of Britain. Coalition ministers warned that, if the currency union was negotiated, it could place contsraints on an independent Scotland's tax policies as well as debt levels.

Yesterday, the row over currency continued as both sides claimed victory in the debate.

Blair McDougall, director of the pro-UK Better Together campaign, said: "The uncertainty over what currency a separate Scotland would use is now a central issue in the referendum campaign.

"Again and again Nicola Sturgeon refuses to tell the Scottish people what her Plan B is for Scotland's currency. They cannot sustain this position for 16 months."

SNP MSP Annabel Ewing said: "The UK Government have admitted that despite all the bluster on currency they haven't ruled out participating in a sterling zone."