The SNP are acting as David Cameron's "little helpers" by targeting Scottish Labour seats and diminishing Labour's chance of forming a government, Jim Murphy has said.
The Scottish Labour leader reaffirmed his party's pledge that there will be no coalition with the SNP and further narrowed the terms under which a looser deal could be agreed.
Labour will not ask the SNP's permission to put plans for a higher minimum wage or zero-hours contracts to the House of Commons, he said.
Last week, UK Labour leader Ed Miliband said he would not move Trident out of Scotland if the SNP demanded it, and insisted the SNP would have no input in a Labour budget.
Mr Murphy told BBC Radio 4's Today programme: "We've been clear, the SNP have been clear, everyone is clear, there is going to be no coalition with the SNP.
"The coalition the Scottish Labour Party wants is a coalition with the English Labour Party and the Welsh Labour Party to have a Labour government, and traditionally the biggest party forms the government in the House of Commons.
"David Cameron can't win seats in Scotland. He has got one constituency in the whole of Scotland, so he needs someone else to win seats for him in Scotland and beat Labour for him.
"And the SNP are playing the role of David Cameron's little helpers.
"If these opinion polls are repeated on election day in Scotland, of course the SNP and the Labour Party will work together, we will work very closely together, but it will be on the opposition benches and we will be looking across at David Cameron as prime minister."
He added: "In Scotland, we are behind in the opinion polls.
"Yes, we are the underdogs... but we can can turn this around. There are polls today which show that in 40 seats in Scotland we're not 17% behind, we're 6% behind so within striking distance, but we've got a lot of work to do."
The SNP has yet to withdraw the offer of a formal coalition but said such a deal is "highly unlikely".
A "confidence and supply" deal where the SNP would support a Labour budget and oppose motions of no confidence or an even looser vote-by-vote arrangement have been mooted as possible alternatives.
The SNP signed a loose "co-operation agreement" with the Greens to prop up its minority Scottish Executive in 2007.
But Mr Murphy said there is "no chance whatsoever" Labour will ask the SNP's permission to put policies to the House of Commons.
"If Labour wins the election, an absolute majority or we are the biggest party, if the SNP wants to vote for our manifesto, that's up to them," he said.
"But we are not going to ask the SNP's permission to put our plans for a higher minimum wage or banning exploitative zero-hours contracts to the House of Commons.
"No chance whatsoever, that is not going to happen."
SNP Westminster leader Angus Robertson said: "Jim Murphy's problem is that Labour were David Cameron's big helpers for two and a half years in the referendum - working hand in glove with them, and since then Labour have voted with the Tories at Westminster for more cuts.
"People in Scotland simply do not buy what Labour is saying, and repeated polls have shown that people trust the SNP more than Labour to keep the Tories out of government.
"If there are more anti-Tory MPs in the House of Commons than Tory MPs, we can lock David Cameron out of Downing Street.
"The Tories could get back into government in these circumstances if Labour let them - the SNP never will - in which case the people of Scotland would never forgive Labour."
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