Former Prime Minister Sir John Major has warned a minority Labour government propped up by SNP votes would be a "recipe for mayhem".
In a speech in the Midlands today, Sir John said it would lead to a "weak and unstable" administration which would wreck Britain's economic recovery.
Ed Miliband accused David Cameron of "threatening the integrity of the United Kingdom" by talking up the prospects of Scottish nationalists in the General Election.
The Conservatives are stepping up their campaign on the dangers of nationalist influence by deploying former Prime Minister Sir John Major to deliver a stark warning that a minority Labour government propped up by SNP votes would be a "recipe for mayhem".
But Mr Miliband insisted there would be no coalition with the SNP and denied Nicola Sturgeon's party would hold the whip hand on policy, telling BBC1's Breakfast:
SNP leader Nicola Sturgeon could present a potentially long-term problem for Labour.
Sir John will give a speech later today saying a Labour-SNP government would be "weak and unstable" and wreck Britain's economic recovery.