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.

With a hung parliament appearing to be the most likely outcome on May 7, and the SNP poised to send a record number of MPs to Westminster, Ms Sturgeon could be in a position of real influence across the UK.

Sir John today claimed the Scottish nationalists represent "a real and present danger'' to the future of Britain, as he urged voters to turn their backs on a minority Labour government propped up by Ms Sturgeon's party.

A partnership between Labour and the SNP would be a ''recipe for mayhem'', with Ed Miliband subjected to ''a daily dose of political blackmail'' from nationalists who would ''create merry hell'' in the hope of promoting the break-up of the United Kingdom, Sir John said.

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."

The SNP leader, who addressed the Scottish Trades Union Congress annual meeting in Ayr, said as the election was so closely contested it gave Scotland the opportunity "to make our voice heard" in arguing for a "better and more progressive politics right across the UK".

She stated: "John Major's comments are silly, over the top and frankly they don't show him in a particularly good light.

"These are the same politicians that during the referendum campaign urged Scotland to lead the UK not leave the UK. Now they appear to say Scotland's voice should only be heard if we say the things they want us to say and vote the way they want us to vote.

"My message to John Major is Scotland's voice deserves to be heard in whatever way the Scottish people choose, and voting SNP means Scotland's voice will be heard more loudly and strongly at Westminster than it has ever been heard before.

"But it will also be a voice for better and more progressive politics at Westminster, so perhaps it's not surprising that John Major doesn't like that. But what he is saying disrespects democracy, it's up to people in Scotland to vote how they choose to vote and make their voice heard."

She claimed the Conservatives were "clearly in panic and desperation" and said: "I want the SNP to go to Westminster to make Scotland's voice heard and also to be a voice for better politics, for the kind of politics that stands in opposition to Tory policies of austerity and undermining public services and pushing more people into poverty.

"I can understand why that message of standing up for ordinary people not just in Scotland but across the UK might offend John Major as an ex Tory prime minister, but I think it's a message that will continue to win support right across Scotland."

Ms Sturgeon said: "I'll leave Tory prime ministers of yesterday to indulge in over-the-top language and I'll continue to campaign positively for the votes and the trust of people in Scotland."