The Conservatives will ban Scottish MPs from having a deciding vote on any changes to the rates of income tax in their first Budget if they win next month's election, David Cameron will announce on Friday.
The prime minister will launch the party's first "English manifesto" as it seeks to keep the focus on Labour's willingness to enter government with the support of Scottish nationalists.
Mr Cameron will say: "Soon, the Scottish Parliament will be voting to set its own levels of income tax - and rightly so - but that has clear implications.
"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.
Labour has traditionally had most to lose from such an arrangement, as it has by far the greatest number of seats in Scotland - although that is likely to change after May's vote.
But the opposition claims Mr Cameron is trying to create a "two-tier Commons" for narrow party advantage. Jim Murphy, the Labour leader in Scotland, last week told reporters the move to create what the Tories are calling an "English income tax" was a "brutal betrayal" of Scotland.
The Tories are keen to keep the focus on any concessions Labour might make to the Scottish National party - which is forecast to win more than 40 seats at the election - in order to stay in power.
The party says the focus on that topic has helped change voters' minds in key marginal constituencies. But a poll released by Panelbase on Thursday showed Labour having consolidated its position in the past week, holding on to 34 per cent of the vote while the Tories have dropped by two points - within the margin of error - to 31 per cent. A Survation poll meanwhile put the Tories four points ahead.
Ed Miliband will on Friday try to broaden the scope of the campaign with his first major speech on foreign policy during the contest. The Labour leader will warn that under Mr Cameron the UK has suffered its "biggest loss of influence in a generation", arguing that a Labour government would seek to play a much more active role in the EU.
Meanwhile, Danny Alexander, the Liberal Democrat chief secretary, will argue that a government consisting only of Labour or the Tories would force up interest rates. He will say the Labour commitment to cutting more slowly, even if that means more borrowing, and the Conservative commitment to holding an EU referendum would both imperil the economy.
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