Cameron, who is fighting for his political survival in the May 7 poll, is promising to pass a law introducing a "tax lock" to prevent increases in income tax, value-added tax or national insurance contributions to state benefits.
His centre-right Conservatives, in power since 2010 as part of a coalition government and at virtual level pegging with the opposition Labour party in opinion polls, has put the economy at the forefront of its campaign.
Labour's finance spokesman Ed Balls criticised the move at a time when, he said, the British economy was "not strong and robust".
"They've obviously decided to do this last minute gimmick because they are losing the argument," Balls told BBC radio.
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Official data released Tuesday indicated that Britain's economy grew much slower than expected in the first quarter of this year. Gross domestic product (GDP) rose 0.3 percent compared to 0.6 percent in the last quarter of 2014.
Labour claims that the Conservatives have a "secret plan" to cut tax credits for families in order to meet their spending targets. Tax credits are a state benefit aimed at people in low-income jobs or caring for children.
The Conservatives have faced criticism from commentators and donors that their campaign has lacked passion, prompting Cameron to inject more energy and rhetorical vigour into his speeches in the last few days.