The Electoral Commission must legally summon Boris Johnson, Dominic Cummings and Conservative officials to give evidence on how the prime minister paid for refurbishments to his Downing Street flat, Labour has said. Calling on the commission to launch a formal investigation, lawyers for the party said the matter was “incontrovertibly in the public interest”. In a letter sent to the commission’s chief executive, Bob Posner, on Sunday and seen by the Guardian, Edwards Duthie Shamash Solicitors said there was “ever-increasing evidence” to warrant such an inquiry. Previously the watchdog has said only that “discussions” are under way with the Tories as they “work to establish” whether any reporting of political donation requirements had been breached. Labour cited Cummings’ incendiary blog post published last week that claimed Johnson planned to “have donors secretly pay” for the refurbishment of the No 11 flat where he lives with fiancee Carrie Symonds and son Wilfred. The bill for the works reportedly ran to up to £200,000. In a post distancing himself from accusations of leaking, the former top aide to the prime minister, who left that role last November, said Johnson’s actions around the refurbishment were “unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations if conducted in the way he intended”. Cummings said he had made known his desire not to be associated with the plan, writing that he refused to help and so his knowledge about it was limited. However, he added: “I would be happy to tell the cabinet secretary or Electoral Commission what I know concerning this matter.” Labour’s lawyers said the commission should look into “potential offences or other actions that may have contravened” the legislation governing political parties’ spending, and said it should summon the relevant invoices to “resolve this”. Cat Smith, a shadow Cabinet Office minister, accused the Conservatives of “trying to cover this up”, and urged them to “hand over everything they have” if they wanted to “do something about the stench of sleaze engulfing them”. If the commission resists launching a formal investigation, some clearer details are likely to be provided in the form of Cummings’ planned appearance in front of a parliamentary select committee next month. He has promised to “answer questions about any of these issues to parliament on 26 May for as long as the MPs want”. The government over the weekend refused to answer questions about whether Johnson had received a loan from a Tory donor to help pay for the works. Liz Truss, the international trade secretary, said Johnson paid all costs personally, but refused to deny Johnson had not originally met the costs, and paid the money only once attention was drawn to the subject. She did not say when Johnson made the payment or whether he had received a loan to help him pay it. Nor did Truss say when the next register of ministers’ interests, which is meant to be published twice a year, would be published, given the last one came out in July 2020. She dismissed the “stream of allegations about personal issues” facing Johnson, and called some of them “tittle-tattle”. A No 10 spokesperson said: “As has been the case under successive administrations, refurbishments and maintenance are made periodically. More information on works on the Downing Street estate, including the residences, will be covered in the Cabinet Office’s 2021 annual report and audited accounts. Any costs of wider refurbishment in this year have been met by the prime minister personally.” They claimed all ministers had “acted in accordance with the appropriate codes of conduct” and added: “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, and gifts and benefits received in a ministerial capacity are declared in transparency returns.” A spokesperson for the Conservatives said: “All reportable donations to the Conservative party are correctly declared to the Electoral Commission, published by them and comply fully with the law.” The Electoral Commission declined to comment.