David Cameron has formally turned down a debate with Alex Salmond on independence, saying Alistair Darling is a better person to go head-to-head with the Scottish first minister.
The decision raises the prospect of there being no televised general election-style debate in the campaign, because the Salmond camp has failed to respond to repeated requests from Darling for a debate.
"It is a well understood and reasonable principle that you get to pick your own team's captain, but not your opponent's as well," the prime minister wrote.
"I understand why you might wish to pursue a diversionary tactic. It is a convenient means of deflecting attention away from the real issues - the lack of credibility of your plans for a currency union, funding pensions and managing volatile oil revenues.
"You want the independence debate to be an argument between you and me; the Scottish government and UK government; the SNP and Conservative party - in fact anything rather than what it really is about. Nor is your argument with the rest of the United Kingdom, it is with the people in Scotland.
"It is for people in Scotland to decide. And it is right for you and Alistair Darling - as the leaders of the respective campaigns, with votes to cast as well as votes to win - to debate head-to-head on TV."
Salmond's office insists the prime minister is the most suitable person to debate him because he is the most senior politician pushing for a 'no' vote.
"The highly political nature of the prime minister's letter rather makes my point for me," Salmond said.
"He is in the impossible position of continuing to enter the debate on Scottish independence without actually being willing to have a head-to-head debate! I would like the opportunity to counter the various spurious and unfounded claims about an independent Scotland he has made in his letter, and the best way to do that is by way of a live televised debate.
"The government in which Mr Cameron serves as prime minister is central to the entire referendum debate from the perspective of the 'no' campaign.
"I have noted the prime minister's apparent unwillingness to take part in another general election debate and I'm sure people will draw their own conclusions from that. Indeed, I believe his refusal to debate Scotland's future with me can be summed up in one word – 'feart'."
'Feart' is a Scottish term signifying fear.
The Better Together campaign reiterated its call for Salmond to debate Darling.
"Alex Salmond has no interest in debating the details of separation. All he is interested in is trying to drive a wedge between Scotland and the rest of the UK," a spokesperson said.
"This is not a battle between England and Scotland, it is a debate on the future of our country. David Cameron does not have a vote in this referendum. Alistair Darling does.
"He is ready to debate Alex Salmond any time and any place. The people of Scotland want details about the impact of the SNP's plans for independence on our jobs, our pensions and our currency. That is why we have already written to the broadcasters asking them to begin negotiations on these debates.
"There is only one conclusion to be drawn from Alex Salmond's repeated refusal to debate Alistair Darling. Alex Salmond is running scared."