Sir Keir Starmer said it was "garbage" to claim he would raise taxes by £2,000 as he traded blows with Rishi Sunak in their heated first TV debate.
The Labour leader initially failed to challenge the prime minister's repeated accusations that Labour's spending plans would cost each family £2,000.
He eventually called it "nonsense" and "absolute garbage", saying his pledge to invest in green projects would result in cheaper energy bills.
Labour said the figure is based on misleading information put out in a "dodgy Tory dossier" and called on Mr Sunak to correct the record.
One of their 11 rebuttals is that the costings rely on "assumptions from special advisors", rather than an impartial Civil Service assessment.
Sir Keir initially struggled to explain this during a debate that saw the pair repeatedly talk over each other, forcing ITV host Julie Ethcingham to intervene and cut them off.
A snap YouGov poll after the clash suggested Mr Sunak narrowly came out on top - with 51% of the audience believing he fared slightly better than Sir Keir.
However, Labour's shadow paymaster general Jonathan Ashworth told Sky's deputy political editor Sam Coates that Labour are leaving the debate "stronger tonight" as he accused Mr Sunak of "lying" about Labour's tax policies.
"Rishi Sunak out of desperation had to collapse into lying in that debate," he said,
"We do not have a plan to tax households in the way in which Rishi Sunak described, and we are not putting up income tax, or national insurance and VAT.
"The only party that has made uncosted commitments in this campaign is Rishi Sunak's party."
Labour has previously said it does not plan to raise personal taxes and its policies are fully costed.
Sunak laughed at over NHS claims
As well as the economy, the pair clashed over the NHS and immigration, with Mr Sunak groaned at and laughed at by the audience on some occasions.
The first rumbling of discontent came after the prime minister was asked how long it would take to fix the "broken" health service.
He pointed to the damage done by the COVID pandemic but said "we are now making progress: waiting lists are coming down".
The Labour leader countered:
"They were 7.2 million, they're now 7.5 million. He says they are coming down and this is the guy who says he's good at maths."
Mr Sunak said NHS waiting times are "coming down from when they were higher", prompting laughter from the audience. He then blamed industrial action, eliciting groans.
"It's somebody else's fault," Sir Keir said.
In another key moment, both were asked directly whether they would use private healthcare if a family member was on a long waiting list for NHS care - with Mr Sunak saying he would and Sir Keir saying he wouldn't.
Immigration debate gets heated
There was also a heated debate over immigration.
Mr Sunak offered his strongest suggestion yet that he could be willing to leave the European Convention on Human Rights (ECHR) if the government's stalled Rwanda deportation plan remains blocked by the courts.
He said:
"If I am forced to choose between securing our borders and our country's security, or a foreign court, I'm going to choose our country's security every single time."
However, he said deportation flights will take off to Rwanda "in July, but only if I'm your prime minister".
"Stick to our plan and illegal migrants will be on those planes - with Labour they will be out on our streets."
Sir Keir hit back:
"The levels of migration are at record highs - 685,000. It's never been that high, save in the last year or two.
"The prime minister says it's too high. Who's in charge? He's in charge. He's the most liberal prime minister we've ever had on immigration."
The Labour leader also said Mr Sunak had "completely failed" to meet his pledge to stop small boats crossing the Channel.
On the issue of the ECHR, he said the UK risked becoming a "pariah" state if it left international conventions.
Who came out on top?
The pair dished out their usual attack lines throughout the debate - with Mr Sunak accusing Sir Keir of having no plan and the Labour leader going in on the Tories' 14-year record in government, particularly highlighting the impact of the Liz Truss mini budget.
A break down of the YouGov polling found that Mr Sunak came out on top in the sections about tax and immigration.
But while he also "won" the debate overall, Sir Keir was victorious in the discussions about the cost of living, the NHS, education, and climate change.
However, in bad news for both leaders, the poll found 60% of people thought the debate was frustrating, compared to 17% who found it helpful and 4% who found it authentic.
Following the heated debate, election campaigning is to take a back seat for a couple of days, as commemorations for the 80th anniversary of D-Day begin.
The leaders' differences will be put to one side as the Normandy landings are remembered.
Both Mr Sunak and Sir Keir will attend the UK's national commemoration event in Portsmouth alongside members of the Royal Family and armed forces veterans on Wednesday, before attention is focused across the Channel for further anniversary events in Normandy.
(c) Sky News 2024: Keir Starmer brands Rishi Sunak's tax claims 'garbage' - but poll suggests PM came out on top