David Cameron's pledge to give English MPs the final say on income tax rates south of the border was denounced by Nicola Sturgeon as a "direct breach" of the prime minister's post-referendum promises to the Scottish people.
Publishing the Conservatives' first English manifesto on Friday, Mr Cameron told voters the party's "English votes for English laws" would be implemented within a year - and would be extended to income tax.
Shortly before the Scottish referendum in September the three main UK party leaders pledged to give more powers to Scotland. The subsequent Smith Commission agreement states: "MPs representing constituencies across the whole of the UK will continue to decide the UK's Budget, including income tax."
Ms Sturgeon, Scotland's first minister, criticised Mr Cameron for his "direct breach of the Smith Commission proposals".
"If there are matters that are genuinely English only, that have no impact in Scotland, I think there's a strong case for Scottish MPs not voting on them," she said on the campaign tour in Glasgow," said the Scottish National party leader. "The problem is there's a lot of issues characterised as English-only issues that are anything but - matters relating to the English health service for example. Decisions taken on that have a direct impact on Scotland's budget."
Adam Tomkins, one of the Conservatives' two political representatives to the Smith Commission, said it was wrong to interpret the deal as a commitment not to introduce restrictions on Scottish MPs' influence over English tax rates. "That was outside our terms of reference," Professor Tomkins said.
MPs from the whole of the UK would continue to decide issues such as what income is taxed and how it is applied, but it was "only fair" that MPs from the rest of the UK have equivalent control over those rates and bands that will be devolved in Scotland, he said.
The SNP surge in Scotland has become a focal point of the Conservative election campaign with the party trying to capitalise on the concerns of voters in England of a post-election deal between Labour and the Scottish nationalists.
The party says the focus on that topic has helped to change voters' minds in key marginal constituencies. "We call him McMiliband now," joked one Conservative minister. "The English don't want to be ruled by the Scots, they believe in the union but they don't want to be held ransom by the SNP."
On Thursday, Gordon Brown joined a chorus of voices from across the political spectrum who warned that Mr Cameron could risk the union by demonising a legitimately elected group of nationalist MPs.
The former Labour prime minister accused the Conservatives of "whipping up anti-Scottish feeling" and becoming "the party of English nationalism".
Mr Cameron on Friday insisted that the Conservatives' "English votes for English laws" was not about undermining the UK but rather about assuring all nations in the union feel they are getting a fair deal.
"[This is] not about division and difference and pulling apart, it is about making our United Kingdom stronger. Because if you have basic constitutional unfairness like we've had, if you have the people in one party of the UK feeling like they are getting a raw deal then resentment festers and that undermines the bonds and the fellow-feeling that are the basis of the United Kingdom."
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>The English manifesto is a culmination of Conservative efforts to give more power to English MPs over English laws at Westminster, having agreed last year to pass more powers to Scotland after the country voted against independence.Under the plans, the Conservatives would ban Scottish MPs from having a deciding vote on any changes to the rates of income tax in its first Budget if the party wins the election.
"Soon, the Scottish Parliament will be voting to set its own levels of income tax - and rightly so - but that has clear implications," Mr Cameron said on Friday.
"English MPs will be unable to vote on the income tax paid by people in Aberdeen and Edinburgh while Scottish MPs are able to vote on the tax you pay in Birmingham or Canterbury or Leeds. It is simply unfair. And with 'English votes for English laws' we will put it right."
The policy forms part of the party's programme to remove voting rights from Scottish MPs on any issue that is passed to Holyrood under the devolution plans being planned by all three parties.
Scottish Labour leader Jim Murphy has also criticised the proposals, describing the "English income tax" as a "brutal betrayal" of Scotland.
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