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.
Sir John Major's warning about the threat the SNP could pose to the UK is an "affront to democracy", Scotland's First Minister said.
Nicola Sturgeon hit out at the former Conservative prime minister's remarks, after he intervened in the midst of a General Election campaign which has put the SNP in the political spotlight.
Ms Sturgeon hit back, insisting: "Some of the comments we're hearing in the media this morning from Tory politicians like John Major are actually an affront to democracy."
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.