Labour is due to finalise its manifesto for the May 6 General Election, with a headline pledge to keep the basic rate of income tax at 20p over the course of the next Parliament.
Prime Minister Gordon Brown let slip the vow during an interview on Wednesday, telling Channel 4 News: "The income tax rate has come down from 23p to 20p and we have kept it at 20p and that is what we will pledge to do in our manifesto."
But the promise was instantly dismissed by Liberal Democrat Treasury spokesman Vince Cable as "a lame gesture" compared to his party's promise to take all earnings under £10,000 out of income tax altogether.
Despite seeing more than 50 business leaders line up behind David Cameron's promise to scrap Labour's planned 1p rise in National Insurance, Mr Brown will keep up his attack on Conservative tax and spend plans for the third day of the election campaign.
Flanked by Chancellor Alistair Darling and Work and Pensions Secretary Yvette Cooper at Labour's first full-scale press conference of the election, the Prime Minister will seek to persuade voters that only he can secure the recovery, while the Tories would put it at risk by removing £6 billion from the economy in 2010 to fund the reversal of the NI hike in 2011. He will later join Labour's National Executive Committee and other senior party figures to approve the party's manifesto for publication next week.
Tory economic plans will also come under attack from Lib Dem leader Nick Clegg, who will accuse Mr Cameron of preparing the way for tax increases.
In a speech in Glasgow, Mr Clegg will say that the reversal of the NI increase, along with promises to raise inheritance tax thresholds and recognise marriage in the tax system, would cost a Tory administration £13.5 billion, while Conservatives have so far identified only £100 million in alternative tax revenue to pay for it.
"Your taxes will go up under the Conservatives," he will say. "It is time for the Conservatives to come clean about how they will pay for these tax cuts... without decimating schools, hospitals and police forces across the country."
Mr Cameron will use his first major press conference of the campaign to highlight his Big Society agenda and promise the creation of a National Citizen Service to give young people the opportunity to volunteer for community work. The Tory leader - who is visiting Norwich and Plymouth - will say he is "so excited" about the scheme, which he will say will be "one of the proudest legacies of a future Conservative government".
Aides said a typical three-week scheme could involve youngsters taking part in a week of team-building outdoor activities, followed by a residential programme looking after elderly people in their area and a chance to develop and run their own social action project.