A GOVERNMENT partnership between Labour and the Scottish National Party following the General Election would be a 'match made in hell' for the UK economy, Prime Minister David Cameron has warned.

Speaking as the SNP launched its manifesto for the May 7 poll, Mr Cameron warned that voters could be 'sleepwalking' towards an outcome which would put government into deadlock and bring recovery to a halt.

He issued an appeal to people considering a protest vote, or those who have previously sworn that they would never back the Tories, to vote Conservative in order to prevent an alliance between "Labour and the SNP - the party that would bankrupt our country and the party that would break up our country".

Mr Cameron's warning came as he set out plans for a new 'Carlisle principle' under which the UK Government would carry out regular reviews to ensure that actions taken by the Scottish administration under devolved powers do not have a detrimental impact on the rest of the UK.

But SNP First Minister Nicola Sturgeon made clear she would resist the plan, telling her party's manifesto launch in Edinburgh: "Let me say this to David Cameron - we will oppose any effort to undermine the Scottish Parliament."

In a bid for support from the regions, Mr Cameron pledged that a Tory government would set a goal that 60% of new jobs should be created outside London and the South-East - a target which he said the coalition had achieved over the past five years.

And he warned that the SNP would use any influence it had over a minority Labour administration to shift investment in jobs, roads and railways north of the border and away from the English regions.

Speaking in Crewe - a marginal Conservative seat which will provide a significant hub for the planned HS2 high-speed railway - Mr Cameron said: "Make no mistake, if Labour and the SNP get into power, you are going to see an alliance between a party that wants to spend, borrow and tax more, with a party that wants to spend, borrow and tax even more.

"It might be a match made in heaven for them but it is a match made in hell for the British economy.

"You would see our deficit climbing up again, spending on welfare soaring again, businesses crushed again, jobs lost again. With Labour and the SNP, our economy will head into ruin again. And who will pay?

"You will pay - in higher taxes."

Mr Cameron added: "Our economic recovery didn't happen by accident, but it could end by accident. I want to appeal to everyone - those wondering how to cast their vote, if you've voted Conservative before or never voted before or swear you'd never vote Tory, please think very carefully about the national outcome you want and how your vote really does affect that outcome.

"Because when you make your mark on that ballot paper, you are writing the future of Britain in permanent ink.

"Labour and the SNP - the party that would bankrupt our country and the party that would break up our country, government in deadlock, our economy heading for ruin. Is that what you want? A vote for anyone other than the Conservatives will deliver it."