ALISTAIR Darling, the head of the pro-UK Better Together campaign, has said he is frustrated by Alex Salmond's continued refusal to face him in a live TV debate about the country's future.

The former Chancellor last night issued a fresh challenge to the First Minister as the referendum campaign approached its final six months, saying: "What does he have to be afraid of?"

Mr Salmond has said he will debate with senior figures in the No campaign only on the condition that David Cameron first agrees to go head to head with him on live television - a proposal ruled out by the Prime Minister.

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But speaking at an International Network of Street Papers event at the Mitchell Theatre in Glasgow, Mr Darling said it would be extraordinary if the stalemate robbed voters of a chance to see campaign leaders face the cameras.

He said: "I think it's absolutely necessary for us to have a debate.

"There is no reason on Earth why we cannot have a debate between two people, both voters in Scotland and both very passionate about the country."

The Labour MP said Mr Salmond wanted to face Mr Cameron in order to turn the independence debate into "him [Mr Salmond] against the Tories".

But he added: "He is the leader of their campaign, I'm the leader of ours. What's he got to be afraid of?

"It would be very odd if we go to the polls without any sort of debate because he would not take part. It would be extraordinary."

The challenge came as the two campaigns prepare to mark the six-months-to-go milestone on Tuesday.

Mr Cameron can expect fresh calls to debate with Mr Salmond today when he arrives in Edinburgh for the Scottish Conservatives' conference.

Mr Darling was speaking during a public conversation with broadcaster James Naughtie at the Mitchell Theatre in Glasgow.

The event was organised by the International Network of Street Papers and part of a series - which has already featured Mr Salmond - sponsored by The Herald and the Fraser of Allander Institute.

Questions from the audience were dominated by the SNP's proposal - ruled out by the main Westminster parties - to forge a currency union between an ­independent Scotland and the UK, if Scots vote Yes.

Mr Darling said the move was in the interests of neither country and insisted it would not happen.

He also dismissed the other possibilities put forward by ­Scottish Government experts - unilaterally using pound, adopting the euro or creating a brand new currency - saying: "They are all terrible options, I don't support any of them."

He said he hoped for a decisive result based on a high turn-out of voters in the referendum.

He said: "I hope we can put to bed this issue of whether we should break away for a generation.

"It won't go away but we can't afford to spend endless years discussing the constitution when issues like health are being pushed to one side because all the discussion is about the constitutional question.

"This is the time we need to make a decision and whatever the result is, we abide by it."

Despite targeting clear victory Mr Darling said he remained nervous about the outcome.

He said: "If you go into any election thinking anything other than it is going to be close you are foolish.

"I'll remain apprehensive, I'll remain nervous right till the time we get the result."