Labour's shadow chancellor Ed Balls said the Tories have a "secret plan" to raise VAT during his economic speech in Leeds on Monday (6 April).
Calling the 2011 VAT rise "David Cameron's first broken promise", Balls said the only way the Conservatives can get their sums to add up will be to raise it again.
"Tory governments always raise VAT. It's the Tory way," he said.
The shadow chancellor also warned that millionaires "will pay less" if the Conservatives win the General Election.
"Let's look at what another five years of the Tories will bring," he said.
"Only yesterday George Osborne repeatedly refused to rule out cutting the top rate of tax again for the very richest. David Cameron refused to rule it out as well.
"We now know this is their secret plan – another big tax cut for millionaires.
"How can this be fair when families here in Leeds and across the country are still struggling and are £1,100 a year worse off on average?"
Balls also said that the Tories could join forces with UKIP if elected.
"UKIP and the Tories stand for the same policies on NHS privatisation and tax cuts for the very richest too.
"No wonder David Cameron and George Osborne won't rule out a Tory-UKIP coalition after the election."
Labour and the Conservatives are going head-to-head over their respective economic plans on Monday (6 April), the official start of the tax year.
The Tories have dubbed the day "money back Monday," with Prime Minister David Cameron due to speak alongside Chancellor George Osborne at a campaign event in the southwest of England, while Liberal Democrat leader Nick Clegg is canvassing in Kingston-Upon-Thames.