A senior minister has insisted Boris Johnson did pay for the refurbishment of his official Downing Street flat out of his own pocket but could not confirm where he had got the money. The international trade secretary, Liz Truss, insisted Johnson paid all costs personally, and dismissed a “stream of allegations about personal issues” in the light of an incendiary blogpost by Dominic Cummings on Friday night that levelled serious claims of impropriety against the prime minister. Ahead of an appearance next month in front of a parliamentary select committee, Cummings said Johnson told him of a plan to “have donors secretly pay for the renovation” to the No 11 flat where he lives with his fiancee, Carrie Symonds, and son Wilfred. Cummings said the plan was “unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations”. The Electoral Commission is investigating, despite the government claiming on Friday that Johnson personally paid the bills – reported to run to up to £200,000. Truss insisted Johnson acted within the rules and tried to play down any public interest in the claims in Cummings’ blogpost, including that the prime minister tried to suppress an inquiry into who leaked news of England’s second national lockdown because it found Henry Newman, a No 10 staffer and friend of Symonds, was responsible. She called the questions on Sky News’ Sophy Ridge on Sunday a “huge distraction” from the success of the coronavirus vaccine programme, and said: “The proper information has been declared in the public domain.” Asked repeatedly when people would get to see the latest register of ministers’ interests, which should be published twice a year but was last released in July 2020, Truss refused to set a date, but added: ““All the details will be given out in line with the rules.” She refused to deny Johnson had not originally met the costs of the flat refurbishment, and paid the money only once attention was drawn to the subject. When asked if she was concerned about Cummings’ claim the arrangement could have been illegal, Truss said she had been “assured the rules have been complied with” – but declined to say by whom. Truss also did not dispute the allegation Johnson tried to suppress an inquiry into who leaked the lockdown announcement, nicknamed the “chatty rat” in Westminster, saying only: “This is tittle-tattle that’s being promoted.” Nor did she rule out Johnson personally briefing against Cummings in the first shot that sparked a fresh civil war between two, after it was suggested the prime minister phoned editors at three friendly national newspapers on Thursday to blame his former adviser for leaking texts from the entrepreneur James Dyson showing he promised to “fix” tax arrangements. “I can’t comment on what newspapers editors and what conversations go on,” Truss said. Labour has demanded a fresh investigation be set up to more rigorously look into the various claims of opacity and impropriety the government faces. Jess Phillips, the shadow domestic violence minister, said it should not be left to “scrapping between two very powerful men”, but an independent inquiry. She stressed Labour was not against all lobbying, but ensuring equal access for companies in her Birmingham Yardley constituency with those of powerful businessmen such as Dyson, and the banker Lex Greensill, whom former prime minister David Cameron lobbied on behalf of. Phillips said she wanted to “take this out of the hands of the people who frankly have questions to answer”, and give it to a parliamentary select committee to make a “totally fair assessment of the situation”. She also made an appeal for Conservative MPs, who voted down the attempt to hold just such an inquiry earlier this month, to reconsider and not be bound by their party whips, saying: “Integrity really, really matters.” The Sunday Times has reported MI5 has been investigating the source of the lockdown leak, and concluded one person sent a message on WhatsApp from the cabinet room after a meeting where it was discussed, with only six people present. There could be further bombshell revelations on Monday, when the cabinet secretary, Simon Case, appears before the public administration and constitutional affairs committee to answer questions on lobbying, where his investigation into senior civil servants’ double-jobbing is likely to be scrutinised. Case’s predecessors, Lord Mark Sedwill and Lord Gus O’Donnell, will appear on the same day in front of a Lords committee to give evidence on updating the cabinet manual, which sets out the main laws, rules and conventions affecting the conduct and operation of government.