Britain's stability would be undermined and the country would face "mayhem" if Labour were to accept a deal from the SNP after the general election, Sir John Major has warned.
The Tory former prime minister issued the intervention when he spoke in the West Midlands today in a bid to boost David Cameron's campaign.
"If Labour were to accept an offer of support from the SNP, it could put the country on course to a government held to ransom on a vote-by-vote basis," Major said.
"Labour would be in hock to a party that – slowly but surely – will push them ever further to the left. And who would pay the price for this? We all would. We would all pay for the SNP's ransom in our daily lives - through higher taxes, fewer jobs, and more and more debt.
"This is a recipe for mayhem. At the very moment our country needs a strong and stable government, we risk a weak and unstable one - pushed to the left by its allies, and open to a daily dose of political blackmail."
The comments come after SNP Nicola Sturgeon launched her party's election manifesto in Edinburgh yesterday.
The first minister reached out to voters in England as she argued the policy document "could bring benefits to the whole of the UK".
Sturgeon added: "A vote for this SNP manifesto on May 7 will make Scotland's voice heard in Westminster more strongly than it has ever been before.
"A stronger voice for Scotland will mean a stronger voice for new, better and more progressive politics at Westminster for everyone."
Ed Miliband, who today will outline Labour's "emergency" recruitment drive to tackle the NHS staffing crisis, has ruled out a coalition with the SNP.
But the Labour leader has left the door open for a confidence and supply (vote-by-vote) deal between the two parties as the polls continue to predict a hung parliament.
The latest survey from YouGov, conducted between 18 and 19 April, put Labour one point ahead of the Tories (35% vs 34%), with Ukip on 13%, the Liberal Democrats on 8% and the Greens on 5%.