Labour is aiming to force a senior minister before parliament this week to account for the growing sleaze crisis engulfing No 10 – amid growing cross-party uproar over a collapse in standards at the heart of government. The Observer understands the opposition is hoping to persuade Lindsay Hoyle, the Speaker, to grant an urgent question on Monday that would mean a senior minister – most likely the Cabinet Office minister Michael Gove – being summoned to the Commons to account for the crisis, explain steps being taken to end it, and take questions from MPs. Demands for action grew on Saturday after Boris Johnson’s former adviser Dominic Cummings made a series of allegations relating to his former boss, including that he had been plotting an “unethical, foolish, possibly illegal” plan to get Tory donors to secretly fund the refurbishment of the No 11 flat in which he lives with his fiancee and their young child. The government has since said Johnson has himself paid the £58,000 bill, but it remains unclear whether he paid directly, or received a loan from the party or a donor. Labour has also raised the question of whether the correct tax has been paid on the refurbishments and any potential gifts. In a letter to the prime minister, the shadow cabinet office minister Rachel Reeves said there should be full transparency around the funding of the refurbishment. “Any external financial aid to a prime minister’s lifestyle must be fully declared at the time and, as the ministerial code makes clear, real and perceived conflicts of interest must be avoided,” she said. The Electoral Commission has said it is still seeking answers from the party over whether any sums relating to the work should have been declared under the law on political donations. At the same time Scotland’s first minister Nicola Sturgeon said nothing less than a full public inquiry was required: “The stench of sleaze that is ssurrounding this Tory government is becoming quite overpowering. There are very serious allegations being levelled at Boris Johnson and his government, including by people who worked closely inside it. As someone who has recently been subject to far reaching inquiries and scrutiny, I say a thorough investigation is needed here, given the range and seriousness of the allegations.” Her party is calling for all of Johnson’s email, text, and call records to be made available for scrutiny. With less than two weeks before local elections in England, and elections to the Scottish and Welsh parliaments, the opposition parties are now determined to increase the pressure on Johnson and the Tories. The latest Opinium poll for the Observer suggest the issue of sleaze is cutting through, although it is not – so far – affecting the parties’ standings. Almost four out of 10 voters think Johnson and the Tory party are corrupt. Some 37% describe Johnson as mostly or completely corrupt, compared with 31% who say he is clean and honest. Even more – 38% – say the Conservative party is corrupt, with 31% saying it is clean and honest. But the poll – taken before news of Cummings’s declaration of war on the prime minister – puts the Tories 11 points ahead of Labour (44% to 33%). The sleaze row will also be centre stage on Monday when the country’s top civil servant is expected to be quizzed over the identity of the person who leaked details of the government’s second national lockdown in England last year. Simon Case, the cabinet secretary, is likely to be asked to verify the claim in Cummings’s blog that Case had concluded the leak did not come from Cummings, but “all the evidence” pointed to Henry Newman, a former adviser to Gove who now advises Johnson. The inquiry into the affair has never been published. No 10 sources have defended Newman against the claim. Case is also set to be asked to confirm Cummings’s allegation that Johnson considered stopping the inquiry because Newman is a friend of the PM’s fiancee Carrie Symonds. Johnson has denied the claim. Cummings has said he has “definitive” WhatsApp messages with “very senior officials” about the issue. He is rumoured to have audio recordings from his time in government, along with reams of WhatsApp messages and emails. Cummings is now understood to be gathering material for a potentially incendiary appearance before a select committee next month examining the government’s response to the pandemic. He is known to have pushed for more severe restrictions to be imposed more quickly than they were. On Saturday, Downing Street, which is understood to have briefed newspapers on Thursday that Cummings was behind the leaks about texts between Johnson and Dyson, declined to repeat the allegation and said it could not comment on matters that were subject to leak inquiries. Senior Tories joined in the criticism with former attorney general Dominic Grieve describing the prime minister as a “vacuum of integrity”. Conservative MPs are furious the party appeared to have provoked Cummings by accusing him of leaking, thereby creating a crisis out of a difficult but manageable series of allegations. One former cabinet minister, now a backbencher, said Johnson had brought the crisis on himself by employing Cummings in No 10 in the first place. “My first thought when I heard of Cummings’s explosion was ‘who could have predicted it?’ The answer is everyone. Everyone told him not to take Cummings into Downing Street because it would end like this. Now Cummings has one and a half years of secrets from the heart of power to spill out at moments of his choosing. All this was completely avoidable.”