Labour mobilised a frantic counterattack on Rishi Sunak on Wednesday as it attempted to rebut misleading claims about potential tax rises, linking the prime minister’s claims to his conduct over Partygate and calling him a liar. There was widespread unease in the party last night, including among shadow ministers and senior MPs, that its leader, Keir Starmer, had not effectively challenged Sunak’s claim that Labour’s spending plans would cause taxes to rise by £2,094, according to what the prime minister said were independent civil service costings. Several MPs said they were horrified by Starmer’s failure to comprehensively deal with the tax allegations during the debate, which one described as the first clear attack that the Tories had landed. Shadow ministers knew they had to get on the attack immediately. “We know we need to deal with it quickly,” said one shadow cabinet aide. On Wednesday, Labour produced a letter from the Treasury permanent secretary, James Bowler, that warned that the tax calculations should not be portrayed as having been produced by civil servants – contradicting Sunak’s claim. That letter now gives Labour MPs and spinners a tool that allows them to say Sunak was misleading. Labour insiders say the exchanges have given them licence to go harder at the prime minister and to use harsher language. They also intend to use the moment as an excuse to bring up Partygate – linking another moment where they will say Sunak was less than straightforward about Covid rule breaking inside No 10. In the aftermath of the debate, Conservative strategists were jubilant that the attack had landed, making the front pages of several newspapers. But by lunchtime on Wednesday, one Labour insider said they had been relieved that the top news bulletins were that Labour had accused the prime minister of lying about civil servants producing the tax figures. Sunak made the damaging claim 10 times in the first 25 minutes of the ITV debate before Starmer addressed it directly. Conservative spinners were relaying their calculations via WhatsApp to journalists and MPs during the debate, which set the narrative quickly about the standout topic of the night and left Labour scrambling to keep up. Pat McFadden, Labour’s election coordinator, appeared so unnerved by the failure to directly rebut Sunak’s claim that he issued his own direct denial on X in the middle of the debate. In the spin room backstage, the shadow Cabinet Office minister Jonathan Ashworth went table-to-table with journalists to label the claims “absolute garbage”, saying that having been a political adviser in the Treasury, he knew how such costings were commissioned. Why had it taken Starmer so long to push back on the claim? One argument was that he had been trying to stick to the debate rules. One Labour staffer conceded privately that the leader had been “maybe a bit too polite”. Internally, there was also frustration at the ITV format, which gave Starmer no time to explain why Sunak’s claim was wrong. “In the time you’ve got, all you can say is: ‘That’s a lie.’ It was annoying Julie [Etchingham] cut him off when he was trying to counter it,” one party insider said. “He did say several times he wanted to answer the prime minister’s claims and she stopped him.” Starmer was prepped on tax claims being a key Conservative attack line, which the party has been using since the chancellor, Jeremy Hunt, held a special press conference in the week before the election was called. By early on Wednesday morning, it was clear to Labour chiefs that they needed a major damage control operation. Many had worked hard in the first hours of the campaign to shut down any notion of Labour tax rises, kicking off the first week with a speech by Rachel Reeves, the shadow chancellor, where she ruled out any further tax rises, including income tax, national insurance and VAT. By Wednesday lunchtime, the party’s spinners had launched eight separate attacks calling the prime minister a liar, including mock-ups of Sunak in the film Liar Liar. Reeves released a video stressing that there would be no new tax rises under a Labour government. Starmer and Reeves both publicly accused Sunak of breaking the ministerial code. Every shadow cabinet minister was deployed across social media to counter the tax claims, including the shadow health secretary, Wes Streeting, who said: “Rishi Sunak lied to you. He’s sent his ministers out to lie to you. They were told not to lie to you. You can’t believe a word they say.” Ashworth said it was “clear … that Rishi Sunak’s strategy is to lie through his teeth … he lied about small boats, he lied about NHS waiting lists, he lied about the cost of living, just like he lied to the British people about his knowledge of the Downing Street parties. This has now become about Sunak’s character and the clear evidence not just that he is hopeless at his job, but a desperate liar.”