PM warns of SNP tie-up with Labour
David Cameron is to raise fresh concerns over the impact of SNP policies on the rest of the United Kingdom.
The Prime Minister will pledge new protections to make sure England, Wales and Northern Ireland do not "lose out" due to decisions taken in Scotland.
The intervention, in a speech in the North West, comes as the Tories attempt to push home their message about the dangers of a tie-up between Labour and the nationalists after May 7.
Mr Cameron has been arguing that Ed Miliband could only get into Downing Street with support from Nicola Sturgeon's party - and may have to appease them by abandoning deficit reduction plans and the renewal of Trident.
During another hectic day of campaigning, the Conservative leader is to insist he will honour the vow made during the Scottish referendum to devolve more powers north of the border.
"But as we go further in devolving powers to Scotland, we need to make sure devolution works for all the other all parts of this country too," he will say.
"Take one example. The SNP government is committed to using its new tax powers to cut and eventually abolish Air Passenger Duty for flights from Scottish airports.
"That could distort competition and see business drawn north of the border, with a huge impact on airports in the rest of the country. So we are reviewing the way Air Passenger Duty works, to make sure other cities don't lose out.
"Today I want to set out a new principle - you could call it the Carlisle Principle - that we will make sure that there are no unforeseen detrimental consequences to the rest of the country from Scottish devolution. For either England, Wales or Northern Ireland."
Mr Cameron will say a Tory government would conduct an annual review of the impact of devolved policies across the UK, with the Chancellor having to report to parliament on how he will mitigate any harm.
"It will look at what effect Scottish government policies are having: whether it's changes to tax rates, business rates, or university tuition fees - or Scotland's powers over energy, agriculture, transport, and public services," Mr Cameron will add.
"To be absolutely clear, this is not about a UK Government stopping the Scottish Government from using its powers as it sees fit or to do things differently," the PM is to say. "It is also not about reopening discussion about the Barnett formula - our commitment to retain as the basis for determining Scotland's funding from the Treasury is clear and unequivocal.
"This is about making sure we understand the impact that devolution is having, and make sure that the rest of the country never unwittingly loses out.
"Decisions made by the Scottish Government can now have a big impact on your job, your income, and investment in your area.
"That's why we will create the Carlisle Principle, and a mechanism to make sure the rest of the country doesn't lose out from Scottish devolution."
But a Labour spokesman insisted the Smith Commission proposals for Scottish devolution already made arrangements to ensure the rest of the UK did not lose out.
"David Cameron is in a panic as the wheels come off the Tory election campaign," the spokesman said. "He has nothing to offer the working people of Britain of any substance - there's nothing new in this announcement today.
"Labour has already said that the implementation of the Smith Commission will need to ensure that the English regions do not lose out, for example on Air Passenger Duty. That remains the case."
Scotland's Deputy First Minister John Swinney said: "The Tories must have given up completely on Scotland.
"Last week the Tories abandoned the Smith Commission, this week they are attacking the Scottish Parliament.
"When the Scottish Government has balanced the books every year, it is insulting to the parliament to say a UK government which has run up £1.5 trillion of debt should check our sums.
"This clearly shows why we need a strong team of SNP MPs to speak up for Scotland and to protect the parliament itself."