Boris Johnson has asked the UK’s most senior civil servant to review how the refurbishment of his Downing Street flat was paid for following damaging allegations from Dominic Cummings. Simon Case, the cabinet secretary, told MPs on Monday that the review would take “a matter of weeks” amid reports that money for the revamp was secretly handed over by Conservative donors. The review has been ordered after Cummings, Johnson’s former top adviser, claimed in a blogpost on Friday that the prime minister’s plans to seek cash from Tory donors to fund the works were “unethical, foolish” and “possibly illegal”. Appearing before a Commons committee on Monday, Case said a review into the refurbishment would look at “how this has been done”. Asked repeatedly whether he was aware if private donations had covered any of the costs, he said he had “not been involved directly in this”, adding: “I do not have all of the facts and details at my disposal.” Case also told the public administration and constitutional affairs committee that a six-month inquiry was unlikely to identify who had leaked details about the autumn lockdown. Reports have claimed that Johnson and his fiancee, Carrie Symonds, have spent as much as £200,000 on renovations in their flat above No 11 Downing Street. Each prime minister receives £30,000 of taxpayers’ money to alter their new home. Labour has called for the prime minister to reveal the full amount spent and who paid in the first place. Speaking earlier, the defence secretary, Ben Wallace, said the prime minister had paid for the revamp “from his own money”. However, Downing Street and the Conservatives did not deny that Tory party funds had been used to pay in part for the makeover. The Spectator magazine reported that Conservative campaign headquarters had footed some of the bill – charged by the Cabinet Office – and Johnson was in the process of repaying his party. A Conservatives spokesperson suggested the loan would be declared, saying: “All reportable donations to the Conservative party are correctly declared to the Electoral Commission, published by them, and comply fully with the law.” On the subject of who leaked details of a planned national lockdown to the press in October, Case told the committee it was probable that investigators would “not successfully” reach a conclusion of who was the culprit, but insisted that inquiries were “ongoing” and would conclude within weeks. Case said he had updated the Commons Speaker, Sir Lindsay Hoyle, within the last fortnight on progress, and promised it wouldn’t be long before MPs could hear of the conclusions. Interest in the leak inquiry rose over the weekend after Cummings, claimed that Case had already exonerated him and that the prime minister had sought to suppress the investigation because he feared blame would be levelled at Henry Newman, a close friend of Carrie Symonds. On Monday, Case confirmed that no inquiries had been discontinued before investigators had finished carrying out their work, and refused to divulge whether he had cleared Cummings of blame. When the leak inquiry was launched late last October following the announcement that England would enter a national lockdown, Case said it had been escalated to the government security group – a Cabinet Office team who he said use a “full range of techniques” to identify the culprit. The Guardian understands that Scotland Yard officers and government officials then met to discuss the material, but it was decided the threshold had not been met for police to become involved. Case said the Crown Prosecution Service decided not to seek a conviction for misconduct in a public office, or under the Official Secrets Act. He did not deny that MI5 were also involved. Government sources said leak inquiries were often “box-ticking exercises” that did not reach a conclusion and were used more as a threat to deter leaking. But they can be escalated to heights seen in the search to identify who leaked discussions from the national security council in 2019. That inquiry ended in Gavin Williamson’s expulsion from the government. He denied any wrongdoing and was subsequently reappointed as a cabinet minister by Johnson several months later. An insider suggested that MI5’s potential involvement in the lockdown leak inquiry – despite the police deciding that no offence had been committed – suggested there were concerns the culprit could have access to national security details that the security services worry could be leaked at a later date, or leave the person open to blackmail. They added that if reports that Cummings had recordings to support his claims of Johnson’s impropriety were correct, it would raise further questions about how much private information he still had access to, despite leaving the government several months ago. Another source said they “bet he has” recordings and commented: “Methink Dominic Cummings doth protest too much.” A No 10 aide also said they “wouldn’t be surprised” if Johnson had rung three friendly newspaper editors to implicate Cummings in a series of leaks, when coordinated briefings appeared in the Sun, Times and Telegraph attributed to unnamed sources last week. Downing Street repeatedly refused to deny the allegation on Monday.