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Sir Keir Starmer has accused Rishi Sunak of "deliberately" lying about Labour's tax plans.
In Tuesday evening's TV debate, the prime minister repeatedly claimed a Labour government would mean £2,000 of tax rises per working household.
The Labour leader said Mr Sunak "knew he was lying” when he made the claim - and he insisted Labour would not bring in tax rises for "working people".
Like the Conservatives, Labour has pledged not to increase the rate of income tax, National Insurance and VAT if it wins the election.
BBC Verify has analysed the costings and found Mr Sunak's £2,000 claim could risk misleading people.
The Tories insist the PM was not lying and Labour's plans would mean higher taxes.
Speaking to the media for the first time since the debate, Sir Keir said: "Last night really mattered because what you saw was a prime minister with his back against the wall, desperately lashing out and resorting to lies - and he knew he was lying. And I don’t say that lightly.”
During the debate, Mr Sunak's claim went unchallenged by the Labour leader for about 20 minutes, although he eventually called them "nonsense".
The prime minister claimed "independent Treasury officials" had costed Labour's policies and they would mean “£2,000 in higher taxes for every working family in our country”.
The Conservatives say this amounts to a £38.5bn spending "black hole", which Labour disputes.
However, on Wednesday morning the BBC revealed the top Treasury civil servant had said the Conservative assessment of Labour's tax plans "should not be presented as having been produced by the civil service".
In a letter to the Labour Party two days ago, Treasury permanent secretary James Bowler said: "As you will expect, civil servants were not involved in the production or presentation of the Conservative Party’s document ‘Labour’s Tax Rises’ or in the calculation of the total figure used."
He added that the £38bn figure "includes costs beyond those provided by the civil service".
The UK’s official statistics regulator says it is looking into the Conservatives' claim.
The Conservatives came up with the number based on how much they say Labour's spending commitments would cost, dividing this by the number of UK households with at least one person working. It covers a period of four years.
During the debate, Mr Sunak suggested the costings had been worked out by impartial civil servants, but they are based on assumptions made by Conservative-appointed special advisers.
For example, one costing looks at Labour’s plan to have more services provided by the state instead of by private companies and it assumes that private companies are always 7.5% more efficient.
But the civil servants doing the costings warned they had "low confidence" in the use of that figure.
Former head of the civil service Lord O’Donnell said costing of opposition party policies was "one of the grubbiest processes I've ever been involved in" and that it had been carried out by both Labour and the Conservatives in the past.
He told BBC Radio 4's World at One programme: "Ministers tell you to produce these costings on some assumptions they give you, which are dodgy assumptions designed to make the policy look as bad as possible."
Conservative Treasury Minister Laura Trott denied Mr Sunak had lied and insisted independent analysis had identified a black hole in Labour's spending plans.
She said: "This is underpinned overwhelmingly by Treasury analysis so if people think Labour are going to win this election they need to start saving."
A Conservative Party spokesperson said: “We were fair to Labour in the production of the Labour tax rise briefing note and used only clear Labour policies, their own costings or official HMT [HM Treasury] costings using the lowest assumptions.”
Liberal Democrat Treasury Spokeswoman Sarah Olney said: “Rishi Sunak’s latest attempts to deceive the British public are nothing new. He has spent years hiking people’s taxes to record levels while claiming to cut them.
“Sunak promised to govern with integrity, but has ended up as a pound shop Boris Johnson.”