Scottish Labour leader Jim Murphy accused the Tories of a “brutal betrayal” of the cross-party agreement on powers for Scotland by proposing a separate English rate.
The Conservatives insisted nothing had changed from proposals unveiled in February giving English MPs a veto on purely English matters at a committee stage, but allowing final votes to include all MPs.
However, Mr Murphy said the manifesto pledge “will fracture the UK tax system and consign Scots to second-class citizens in the House of Commons”. It demonstrates that “you can never trust a Tory” and that “no promise is solemn enough not to be broken” by Labour’s former Better Together partners, he said.
The clash came after Mr Cameron unveiled his party’s general election manifesto, which included a pledge to legislate to prevent the 1.4 million people in the UK and 150,000 Scots on the minimum wage from paying any income tax.
Speaking on a visit to a key marginal seat in Swindon, Wiltshire, the Prime Minister promised “a good life” for British voters, branding the Tories “the party of the working people” and insisting a Conservative government is the only way to stop “the horror” of a Labour government backed by the SNP.
The manifesto states that the Tories will “extend the principle of English consent to financial matters such as how spending is distributed within England and to taxation – including an English rate of income tax – when the equivalent decisions have been devolved to Scotland”.
The Smith Agreement on Scottish devolution states: “MPs representing constituencies across the whole of the UK will continue to decide the UK’s budget, including income tax, given that income tax will still apply on a UK-wide basis.”
Mr Murphy said: “This is a brutal betrayal of Scotland and the Smith consensus. This is the end of the Smith consensus.
“This whole thing stinks – it is the official banning of Scottish Labour MPs from the UK Budget.
“In an attempt to appeal to Ukip voters in Thanet, the Tories are ripping up the centuries-old UK income tax system.
“Scotland is now in danger of being caught in a classic pincer movement between a Tory party that wants to cut Scotland out of the UK budget, and the SNP that wants to cut Scotland out of UK taxes.
“It’s a pretty dramatic development. It’s belligerent, it’s brutal and it’s a betrayal.”
Mr Cameron said: “Labour has written a very thin manifesto with very little detail. The SNP are now going to write what I suspect will be a more detailed list which they will then try to enforce on to an Ed Miliband government.
“And here is the point of this election – if you want to stop the horror of an Ed Miliband government backed by the Scottish National Party, it’s no good voting Liberal Democrat, they could help make it happen. It’s no good voting Green, it’s no good voting Ukip.
“You have to vote for the Conservative Party as the only party that can secure a majority government to keep Britain on the right track.”
He added: “We will put all that at risk if we go back to the old Labour ways of more borrowing, more spending, more out-of-control welfare, more debt and more taxes.
“I don’t think they have learned anything from the last five years and their manifesto gives them the leeway to do it all over again. It won’t take many prods from Nicola Sturgeon and Alex Salmond to make them do it all over again but, believe me, if there were some it would be even worse.”
Scottish Conservative leader Ruth Davidson pointed out that Mr Murphy had been publicly embarrassed by the UK Labour leadership on Monday when he was contradicted over promises of no more spending cuts.
She said: “Scottish MPs will still vote on the Budget and will still vote on the reserved elements of income tax, such as the personal allowance and the rate on savings and dividends.
“This is a desperation ploy from Scottish Labour after Jim Murphy got a smack down from his London colleagues yesterday. Nobody will be taken in by it.”
Mr Cameron told party members the task of the next parliament is “to build on the foundations” of the economic recovery.
His flagship policy for the UK as a whole was to pass a law that means the point at which income tax will be paid will automatically rise with increases in the minimum wage, to make sure less well-off people working full-time will no longer pay income tax.
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