Ed Miliband and Nicola Sturgeon are "at the altar, and preparing to sign the register" on a post-election deal, according to Scottish Conservative leader Ruth Davidson.
Sturgeon made the Labour leader an offer to join forces to help him become Prime Minister during the TV debates.
But David Cameron has warned there would be more borrowing and higher taxes if Miliband goes ahead with the deal.
David Cameron has claimed he was not invited to last night's televised leaders debate.
Asked by ITV News' James Mates why he had "ducked out" of the event, the Prime Minister said, "Last night was a 'challengers' debate'."
"It was the idea of the broadcasters to have a debate of the opposition parties, a debate to which Nick Clegg and I weren't invited," he continued.
Cameron was heavily criticised after he refused to sign up to the broadcasters' original plan of three debates.
The Ukip leader said he did not "lose my rag" when he attacked the audience during last night's televised debate for their "left-wing bias."
We watched undecided voters as they watched the debate hoping it might help them make up their minds.
Ed Miliband appeared to turn down Nicola Sturgeon's latest offer as the leaders of five "challenger" parties took part in a heated debate.