A furious Boris Johnson tried to fight off allegations he broke donation reporting rules, as Sir Keir Starmer branded him “Major Sleaze” in the “cash for curtains” row increasingly engulfing the prime minister. An hour after the Electoral Commission launched an investigation and said there were “reasonable grounds” to suspect payments for renovations to Johnson’s Downing Street flat could constitute several offences, the prime minister was accused of focusing on petty personal issues instead of the pandemic. Starmer said Johnson had been found to be nipping out to choose wallpaper at more than £800 a roll and phoning newspaper editors to “moan” about his former adviser Dominic Cummings, and accused the government of being “mired in sleaze, cronyism and scandal”. In a heated clash at prime minister’s questions on Wednesday, Johnson said Labour’s interest in whether he was given or lent £58,000 to pay for a makeover to his residence was “absolutely bizarre”, and said the “credulity of the public” was “strained to breaking point” with Starmer’s questions. Johnson repeatedly refused to deny that last year he was given the cash – which has not yet been published in any declarations. He insisted he had repaid sums to the Cabinet Office “personally”, but dodged calls to say who had footed the initial bill. Tory MPs have been privately increasing pressure on Johnson to come clean over whether a Tory peer helped fund the works to the No 11 residence he lives in with his fiancee, Carrie Symonds, and their son, Wilfred. Starmer asked why Britain’s most senior civil servant, the cabinet secretary, Simon Case, had been asked to look into the source of the payments. “Why doesn’t the prime minister just tell him? That would be the end of the investigation,” the Labour leader inquired. Johnson said: “I have covered the costs, I have met the requirements that have been advised to me in full.” He added: “Any further declaration I have to make, if any, I will be advised upon.” But Starmer said the public would be screaming at their televisions in exasperation at Johnson refusing to directly answer the questions. He added that the Conservatives had been found to be handing out “dodgy contracts”, “jobs for their mates” and “cash for access”. He labelled Johnson “Major Sleaze” – a retort to the prime minister’s moniker for him of Captain Hindsight" – and said the public deserved a prime minister they could trust. Raising his voice so the sound from the microphone in front of him became distorted, Johnson insisted he was “getting on with delivering on people’s priorities”, and lamented: “He goes on and on about wallpaper.” Johnson also repeated his denial that he said just before deciding to order England’s second national lockdown: “Let the bodies pile high in their thousands.” He called on Starmer – and the Scottish National party’s Westminster leader, Ian Blackford, who asked if he was a “liar” – to publish the identities of the multiple sources, to substantiate the claim. The “cash for curtains” row – as it has been labelled by some in Westminster – exploded over the weekend, when Cummings published an incendiary blogpost that made several claims of impropriety against Johnson. Cummings said he had told the prime minister “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”. A Conservative party spokesperson insisted, after the Electoral Commission inquiry was launched, that “all reportable donations have been transparently and correctly declared and published”, and said they would “work constructively” with the watchdog. Johnson’s press secretary said the prime minister would cooperate fully with the inquiry but that he had not been contacted for information thus far and added the inquiry is into the party’s actions. She declined to comment on whether Johnson or his fiancee would hand over evidence, including phone messages, if asked. “It’s a matter for Conservative party as a political party … they will provide all necessary information to assist the commission,” she said. Also investigating the issue will be the government’s new adviser on ministerial standards, Christopher Geidt. His post had been vacant for months, following the resignation of Alex Allan, who quit when Johnson overrode his finding that the home secretary, Priti Patel, broke the ministerial code by bullying staff. Lord Geidt, who was private secretary to the Queen for a decade until 2017, will begin an immediate inquiry into the flat refurbishment payments.