David Cameron would be able to "waltz" into Downing Street after the general election if Labour and the SNP do not strike a coalition government deal, according to Nicola Sturgeon.
The First Minister of Scotland said it would be an "odd move" from Ed Miliband if he ruled out a pact with her nationalist party.
"If Labour were to rule out working with the SNP, which is a progressive party, they would kind of be saying they would be happy standing back and watch David Cameron waltz back in to Number 10," Sturgeon said.
"I think a lot of people in Scotland would find that very difficult to take, so I watch with interest. I think it would be an odd move for Labour to make to rule out any kind of working with the SNP because it would be signalling that they would rather see the Tories back in government."
The SNP leader made the comments on ITV's Lorraine show after a report claimed that Labour were close to ruling out forming a coalition with the SNP that involves ministers from Sturgeon's party.
But the Guardian article said that Labour argued that it is impossible to rule out any "looser relationship" without "questioning the legitimacy of Scottish MPs in Parliament".
"David Cameron for his own electoral purposes is trying to suggest Scottish MPs should have no vote at Westminster and that is an extraordinary position for a Unionist politician to adopt," the paper quoted one Labour source as saying.
Sturgeon travelled to the capital today to speak at the London School of Economics (LSE), where she told students that Westminster needs an "overhaul".
"The Palace of Westminster is in such a dilapidated condition. Its fabric is crumbling, some areas are prone to flooding and other parts haven't been properly refurbished for generations," she said.
"It's not just the building that needs to be overhauled. It's the institutions."
The latest poll from YouGov, which questioned more than 1,600 voters between 12 and 13 March, put Labour and the Tories neck-and-neck (34% vs 34%) as the election looms.
But the SNP's surge north of the border means Sturgeon's party could hold the balance of power after May.
A Ipsos Mori poll for STV News, which questioned more than 1,000 Scottish voters, found that SNP had a 28-point lead over Labour (24% vs 52%).
The data means that the SNP could take a horde of seats from Labour and win 55 constituencies against Labour's projected four.