As the ongoing row about TV debates on September's independence vote escalated, Deputy First Minister Nicola Sturgeon accused Mr Cameron of snubbing Scotland. She was responding to reports that Downing Street is drawing up a "2-3-5" plan for TV debates before next year's General Election.
Under the proposal, Mr Cameron would be prepared to hold one head-to-head debate with Labour leader Ed Miliband, as the two potential prime ministers, and one also including Liberal Democrat leader Nick Clegg. A third debate would feature the three main party leaders plus Ukip leader Nigel Farage and Greens leader Natalie Bennett.
In the run-up to the 2010 election the SNP failed in a legal bid to block a BBC debate between then-Prime Minister Gordon Brown, Mr Cameron and Mr Clegg from being shown in Scotland unless Alex Salmond could also take part.
But yesterday the SNP said its priority was to secure a referendum debate between Mr Salmond and Mr Cameron. Ms Sturgeon, who is also the "Yes minister" responsible for the Scottish Government's independence strategy, said: "It's astonishing that David Cameron is happy to have a head-to-head debate with Nigel Farage - whose party doesn't have a single seat in the House of Commons - but continues to run scared of a debate on the future of Scotland with First Minister Alex Salmond."We now know he is happy to debate with the leader of Ukip, who constantly lose their election deposits in Scotland, but is not willing to debate with Scotland's democratically elected First Minister.
"That begs the question: what is he afraid of?" She added: "It was reported that the Prime Minister is to visit Scotland more often over the next few months in order to campaign for a No vote. Why on earth is he not happy to publicly debate his position? Given he is now happy to debate with a wide range of politicians in England, the Prime Minister should stop snubbing Scotland."
A Coalition Government source said: "Nothing has changed. The referendum is a debate between Scots with differing views on what is best for Scotland. The TV debates should be between people with a vote - that means Alistair Darling and Alex Salmond."
Mr Cameron has always refused a TV debate with Mr Salmond as he does not have a referendum vote. Mr Salmond refused a debate with Mr Darling, leader of the cross-party Better Together campaign, unless the Prime Minister agreed to go head-to-head at some point before polling day. The stalemate has raised the possibility that Mr Salmond will not feature in a live TV debate before the referendum.
There is no guarantee Mr Cameron's plan for TV debates before the General Election in May 2015 will come to fruition. The final format will depend on discussions between the parties. Downing Street does not want to start talks until after the party conference in October. Labour says discussions should start now.
The latest row came as the SNP accused the Scottish Tories of planning to "drown out Scotland's voice at Westminster" if Mr Cameron returns to power next year. The claim followed comments by Professor Adam Tomkins, an adviser to the Scots Tories' devolution commission investigating possible further powers for Holyrood.Writing on his blog he said Scotland's "over-representation" in the Commons would have to be tackled if the Scottish Parliament assumed significant new powers. 59 of the 650 Westminster MPs are sent from Scotland, a cut from 72 since the Holyrood parliament was established.
SNP MSP Aileen McLeod said: "Ruth Davidson [the Scottish Conservative leader] must now confirm whether or not she agrees with her adviser and come clean on her party's plans to weaken Scotland's hand at Westminster if there was a No vote."