Nicola Sturgeon has criticised the Prime Minister after he claimed an alliance between the SNP and Labour would be the "'ultimate nightmare scenario'' for Britain after the general election.

The Scottish First Minister, speaking during a visit to London, responded: "People in Scotland should think that anything that is a nightmare for David Cameron is a good thing for most other people."

She spoke out after Mr Cameron used his speech to the Scottish Conservative conference last week to warn: "A vote for the SNP is a vote for Labour in government.''

The Tory leader claimed that a "vote for anyone other than the Conservatives risks Ed Miliband becoming prime minister, leading to an unstable minority government".

Polls suggest the SNP could be on track to win a record number of seats at Westminster in May, with backing for Ms Sturgeon's party having surged in the wake of last September's independence referendum.

At the same time, Labour faces the prospect of losing a number of seats in its Scottish heartlands, reducing Mr Miliband's chances of securing an overall majority.

Ms Sturgeon, who will celebrate her 100th day as First Minister on Friday, said it was "unlikely" the SNP would enter a formal coalition with Labour in the event of a hung parliament, but said they could work with them on an "issue-by-issue basis".

She said: "Clearly the SNP is riding high in the opinion polls and we're looking forward to the general election.

"If the SNP performs strongly in the general election Scotland's voice will be heard much more loudly in the Westminster system, and that's got to be a good thing, and perhaps we wield some influence in the Westminster system and look to make alliances with others across not just Scotland, but rest of the UK, to pursue progressive politics, an alternative to austerity, a focus on education and health rather than putting new nuclear weapons on the River Clyde at an exorbitant cost.

"These are things we're looking to pursue at Westminster and I'm sure there are plenty of voices across the UK who will join us."

She continued: "It's unlikely the SNP would go in to a coalition with Labour in a formal sense, we certainly won't do anything that props up a Conservative government, we're more likely to work with a minority Labour government on an issue-by-issue basis.

"What we won't do is support a Labour government that just implements Tory policies.

"The point of SNP MPs in Westminster is firstly to stand up for Scotland and to make our voice heard, but also to pursue progressive politics and progressive policies, and that's the kind of approach we'll take if people in Scotland vote SNP."