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.
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.
In a letter to party supporters she said: "In the last few days, we've now seen hard evidence that this nightmare scenario is now all too real.
"Another poll of Scottish seats on Friday showed that Scottish Labour is now a sinking ship, and is losing voters to the SNP. In truth, there is no way back for them.
"Their weakness means that one thing has become near inevitable: if Ed Miliband is in the position to form a government on May 8th, he will need the SNP's support to push him over the line.
"I said at the turn of the year that Ed Miliband and Nicola Sturgeon were halfway down the aisle towards a deal.
"The truth is that they are now at the altar, and preparing to sign the register. It's the only way Labour can get in to power."
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.