David Cameron will today announce a new "Carlisle principle" to ensure that Sottish government decisions do not have a detrimental effect on the rest of the UK.

The Conservative leader will say that it is important the rest of the country does not "lose out" through Scottish devolution.

Tory sources stressed that the move would not affect Scottish ministers' ability to make policy decisions.

Instead, action could be be taken in England, Wales or Northern Ireland to balance out the effect, they said.

But the announcement comes at a time of strained relations between London and Edinburgh.

The Tories are set to step up their rhetoric against the SNP, with former conservative prime minister John Major to warn that the Nationalists could break up the United Kingdom.

Last week Scottish Labour leader Jim Murphy also accused Mr Cameron and his party of "betrayal" after the Tory manifesto included plans for an English rate of income tax.

Labour figures also accused the Prime Minister of "desperate" tactics and "lies" after he claimed that Ed Miliband had stated that the only way he could become Prime Minister is with the help of the SNP.

The Labour leader has always insisted that he is campaigning for an overall majority in May's election.

In a speech in the North West, Mr Cameron will say that a Conservative government after May would conduct an annual review of the impact of the Scottish government's policies, including over taxation, to assess if they are having an adverse impact on other parts of the uk.

He will say that the "Carlisle principle" - named to reflect the sort of areas that could be affected - will in no way prevent Scottish ministers from making decisions in devolved areas.

Coalition government ministers have already commissioned a review into the impact of the Scottish government's new powers to cut air passenger duty on airports south of the border.

But Mr Cameron will tell voters in England, Wales and Northern Ireland that Scottish government decisions can now have an impact on "your job, your income, and investment in your area".

He will say: "In the Scottish referendum, we made a clear promise to devolve more powers to the Scottish parliament.

"After we won that referendum, we kept our promise, and agreement was reached - for the first time - between all five of Scotland's major parties to give the Scottish parliament extensive new powers. We did that because as Conservatives we believe in decentralisation and decisions being taken as close to the people they affect.

"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.

"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.

"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 rest of the country never unwittingly loses out."

Mr Cameron will also set out an 'ambition' to have three out of five new jobs created outside London and the South East.

The Tories have stepped up their campaign to warn that an SNP government would be "chaos".

But Mr Cameron refused to rule out a post-election deal with Ukip yesterday, despite repeated questioning.

The Tory leader failed to back up comments from his party's chief whop michael Gove, that it would not get "into bed" with Nigel Farage's Eurosceptics.

Lib Dem leader Nick Clegg challenged to Mr Cameron to publicly rule out an telling him not to treat voters as if they are "stupid" by claiming the Tories are on course to win a majority.

In an appearance on BBC1's Andrew Marr show, Mr Cameron declined to rule out any post-election deal with Ukip if there was a hung parliament.

Scottish Conservative leader Ruth Davidson has said that Ed milliband and Nicola Sturgeon are "at the altar, and preparing to sign the register" on a post-election deal.

In a letter to party supporters she described the prospect of a labour government supported by the SNP as "outright chaos".

The Tories have claimed that a Labour-SNP alliance after may would be "chaos".