Dominic Cummings has launched an unprecedented and extraordinary attack on Boris Johnson, alleging that the prime minister tried to quash a leak inquiry as it implicated an ally, and hatched a “possibly illegal” plan for donors to pay to renovate his flat. The outburst by Cummings, a day after anonymous No 10 sources claimed that he had leaked private text messages between Johnson and the billionaire James Dyson, prompted Labour to accuse the government of “fighting each other like rats in a sack”. Cummings used a lengthy post on his personal blog to deny any leaking. Instead, he accused Johnson and his team of a series of wrongdoings. He said the prime minister had behaved in a way he considered “mad and totally unethical”, and warned that he would happily give evidence under oath to an inquiry. “It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves,” he wrote. Such a damning intervention by the man who was Johnson’s key ally and ideological inspiration will deeply alarm the prime minister and his aides. Cummings is due to give evidence to MPs next month. Cummings, who left Downing Street in November, dismissed the accusation, in an anonymous briefing to several newspapers on Thursday, that he had leaked the texts between Dyson and Johnson. In the exchanges last March, the prime minister appeared to promise the businessman that he would “fix” an issue on the tax status of Dyson staff working in the UK during the pandemic. Cummings said he had checked his phone and had not been forwarded the messages in question. He claimed he had been told by Downing Street officials that Dyson’s office had emailed screenshots of his exchanges with Johnson to a series of officials, including some at the Treasury, and that this was what had been leaked. He said he had not been copied into this. “I am happy to meet with the cabinet secretary and for him to search my phone for Dyson messages,” he wrote. “If the PM did send them to me, as he is claiming, then he will be able to show the cabinet secretary on his own phone when they were sent to me. “I am also happy to publish or give to the cabinet secretary the PM/Dyson messages that I do have, which concerned ventilators, bureaucracy and Covid policy – not tax issues.” Cummings also addressed reports suggesting he had been the serial leaker known as the “chatty rat”, who had also allegedly leaked news of another Covid lockdown last autumn. In perhaps the most potentially devastating allegation in his blogpost, Cummings claimed that in a meeting after the leak, the cabinet secretary, Simon Case, told him and Johnson that “all the evidence” pointed to Henry Newman, then an adviser at the Cabinet Office, who has since moved to No 10. Newman is known to be close to Carrie Symonds, Johnson’s fiancee, seen as a key figure in Cummings’ removal from his job. Cummings wrote: “The PM was very upset about this. He said to me afterwards: ‘If Newman is confirmed as the leaker, then I will have to fire him, and this will cause me very serious problems with Carrie as they’re best friends … [pause] Perhaps we could get the cabinet secretary to stop the leak inquiry?’ “I told him that this was ‘mad’ and totally unethical, that he had ordered the inquiry himself and authorised the cabinet secretary to use more invasive methods than are usually applied to leak inquiries because of the seriousness of the leak. I told him that he could not possibly cancel an inquiry about a leak that affected millions of people just because it might implicate his girlfriend’s friends.” Cummings did not give any further explanation of what he meant by the “more invasive methods”, or whether they had been used. He said he had warned some officials about Johnson’s plans, and that they would give evidence under oath to an inquiry, adding: “I also have WhatsApp messages with very senior officials about this matter which are definitive.” On Friday night, No 10 said: “The PM has never interfered in a government leak inquiry.” Finally, Cummings said he had warned Johnson about renovations to his Downing Street flat costing a reported £58,000, for which the prime minister had allegedly sought outside funding from Conservative supporters. He wrote: “I told him I thought his plans to have donors secretly pay for the renovation were unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations if conducted in the way he intended… I refused to help him organise these payments.” Cummings said Johnson had stopped speaking to him about the issue in 2020 after he said this, adding: “I would be happy to tell the cabinet secretary or Electoral Commission what I know concerning this matter.” He also accused the new head of communications at Downing Street, Jack Doyle, of having given the briefing to newspapers on Thursday. Earlier, the government sought to close down the renovations controversy by releasing a statement saying no outside finance had been involved. The statement, released on Friday by a Cabinet Office minister, Nicholas True, revealed that contractors had been brought in to paint, sand and refresh floorboards. But Lord True added: “Any costs of wider refurbishment in this year have been met by the prime minister personally.” After the release of Cummings’ blog, No 10 responded: “At all times, the government and ministers have acted in accordance with the appropriate codes of conduct and electoral law. Cabinet Office officials have been engaged and informed throughout and official advice has been followed. “All reportable donations are transparently declared and published – either by the Electoral Commission or the House of Commons registrar, in line with the requirements set out in electoral law. “Gifts and benefits received in a ministerial capacity are, and will continue to be, declared in transparency returns.” Cummings had written the issues needed to be handled by “an urgent parliamentary inquiry into the government’s conduct over the Covid crisis”. He concluded: “Issues concerning Covid and/or the PM’s conduct should not be handled as No 10 has handled them over the past 24 hours. I will cooperate fully with any such inquiry and am happy to give evidence under oath.” Angela Rayner, Labour’s deputy leader, said the government had “spent the last 24 hours lurching between cover-ups and cock-ups”. She added: “The Conservatives are fighting each other like rats in a sack and slipping deeper and deeper into the mire of sleaze. It shows breathtaking contempt for the country.”