Better Together indicated that Mr Darling would not take part in the televised debate, claiming that STV had revised plans to stage the clash on 16 July, moving it until after the Commonwealth Games at the behest of Mr Salmond.
At the weekend, it was reported that Mr Darling was lined up to take part in the debate with the SNP leader on 16 July.
Mr Salmond indicated he was prepared to debate with Mr Darling if Prime Minister David Cameron continued to refuse to come to Scotland to argue for the No camp.
At the weekend, Mr Salmond’s spokesman said the First Minister would continue to pursue MrCameron to agree to the 16 July date. The First Minister’s spokesman added that if the Prime Minister continued to “duck” the encounter, Mr Salmond would take on Mr Darling after the Commonwealth Games, which end on 2 August.
Mr Cameron has repeatedly refused Mr Salmond’s challenges to a head-to-head debate, arguing that this would turn the referendum into a contest between England and Scotland.
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The Prime Minister has said he should instead go up against Mr Darling, arguing that as the Better Together leader and a Scottish resident who is entitled to vote in the referendum, the former Labour chancellor is the most appropriate choice.
Last night, Better Together insiders claimed STV had agreed to postpone the debate until after Glasgow 2014, despite the broadcaster previously saying the July date was “non-negotiable” during discussions to set up the two-hour programme in front of an audience of 500.
Although Mr Darling has agreed to take part in a similar televised debate on the BBC on 12 August with Mr Salmond, Better Together sources said the No campaign leader would not co-operate with STV’s new date.
Last night, a Better Together campaign insider said: “This is astonishing stuff. STV told us that they would not negotiate on the date of this debate. They were emphatic that the date would not change.
“That hardline stance lasted until Salmond decided that, even although he was free on that date, he didn’t fancy debating Alistair. STV have simply rolled over. It is not on.
“We are desperate to have this debate. The public are desperate for it. The only person stopping it happening is Alex Salmond – but he can’t run away forever. We have accepted an invitation from the BBC for a debate on 12 August. We hope that this one goes ahead.”
Better Together is understood to regard the change of date as a sign that STV appear willing to negotiate with the Yes campaign, but not with Better Together. The No campaign also argues that if Mr Salmond is available to debate the Prime Minister on 16 July, his diary should be able to accommodate a debate with Mr Darling in the Prime Minister’s absence.
Last night, the First Minister’s spokesman said: “This outburst at STV from the No campaign shows they are rattled and fraying at the edges.
“Our position is clear and has been throughout – we will pursue the Prime Minister for a head-to-head debate on STV’s proposed date of 16 July or in advance of the Commonwealth Games.
“And the only thing stopping a debate on that date is David Cameron’s refusal to step up to the plate. If he continues to duck the debate, we will take on Alistair Darling after the Commonwealth Games.
“We are grateful to the No campaign for drawing attention to the fact the Prime Minister is feart of debating Alex Salmond.”
A spokeswoman for STV said: “STV is firmly committed to staging a live TV debate between the First Minister and Alistair Darling. “We have not received agreement from both sides on the proposed date of 16 July and are reviewing the proposed date on that basis.”