David Cameron's claim that Labour would impose a £3,000 tax rise on every working family has been branded "obviously ridiculous" by Newsnight's Evan Davis.
The BBC presenter and former economics editor said the prime minister's attack on Ed Miliband, made outside No.10 Downing Street as he launched the formal election campaign, had been "thoroughly demolished" within hours by the independent think-tank the Institute for Fiscal Studies (IFS).
The IFS said even if Cameron's claim that Labour would raise taxes by £15bn from 2017–18 onwards was true - the tax rise for individual households should be expressed as an annual figure of £560. The prime minister had instead chosen to talk about a tax rise of £3,028 - the cumulative figure over the entirety of the next five year parliament.
And the IFS also suggested the annual tax rise under Labour, if it even happened, could actually work out to be closer to £100 per working household per year.
In a lengthy grilling of Grant Shapps on Monday evening, Davis could barely contain his irritation with the Conservative Party chairman.
Davis said the Conservatives had chosen a "ridiculous way of calculating" a supposed Labour tax rise. "Everybody respectable does it per year," he said.
"Is this how you're going to fight the rest of your campaign?" Davis asked. "It is obviously a ridiculous figure. The IFS are quite sensible, quite independent. They wouldn't say a much lower figure if it wasn't a better figure to use, they don't say that because they know this figure is ridiculous."
He added: "Who do you think the public should believe? Should they believe you, or should they believe the independent fiscal experts of the IFS?"
Shapps defended the Conservative Party's sums and the decision to speak of a cumulative five year number rather than an annual one. He said if Labour won power then "over the life time of the next parliament, you will pay more than £3,000 if you are a working family".
Cameron launched the election yesterday with a short speech outside No.10 after visiting the Queen. In an unusual move for the setting, the prime minister chose to attack Miliband by name. "In 38 days' time you face a stark choice. The next prime minister walking through that door will be me or Ed Miliband," he said.
"You can choose an economy that grows, that creates jobs, that generates the money to ensure a properly funded and improving NHS, a government that will cut taxes for 30 million hard-working people and a country that is safe and secure.
"Or you can choose the economic chaos of Ed Miliband's Britain - over £3,000 in higher taxes for every working family to pay for more welfare and out-of-control spending. Debt will rise and jobs will be lost as a result."
The £3,000 claim was dismissed by Labour as a "made-up figure", and the IFS analysis which found that the party had not said "anything to suggest" that this is what they are planning. "There is little value in bandying around numbers which suggest either party would increase taxes by an average of £3,000 for each working household," the think-tank said in a statement.