Labour leader Ed Miliband exchanged personal insults with the Prime Minister as part of the ongoing row over if and when TV debates should be held.
Mr Cameron branded the Labour leader “weak and despicable” as he denied he was “chickening out” of the events.
At their weekly PMQs exchange in parliament, Mr Miliband said the PM should “admit the truth..he is worried he might lose again”.
He insisted he would be attending the debates on the dates allocated by the broadcasters, April 2 and 16.
But Mr Cameron said: “He said any time, any place, anywhere. I told him March 23 - let’s hold that debate.
“I’ll tell him what has changed. We have now got a situation where it is obvious Labour cannot win without the SNP.
“He says we need the two leaders who can call the tune. That is me and Alex Salmond, so let’s have the debate.”
Mr Cameron said Labour was not hoping to win an outright majority any more. “They are just trying to crawl through the gates of Downing Street on the coat tails of the SNP,” he said.
“What he has got to do is prove he is not a chicken and rule that out.”
Nearly three-quarters of voters believe broadcasters should go ahead with TV leaders’ debates even if Mr Cameron refuses to take part, according to a poll. Only 13 per cent said the debates should not take place without the Prime Minister.
Some 64 per cent said in a ComRes poll that Mr Cameron had damaged his reputation and 59 per cent that he was behaving in a “cowardly” way. 63% said broadcasters should leave an empty lectern to mark his absence.