Labour has formally sought an explanation from the government as to why there has been no investigation into whether Matt Hancock broke the law during his relationship with an aide, despite what the party called “clear evidence” of a breach. The Labour request, in a letter to the attorney general, Michael Ellis, comes as a political thinktank argued that Hancock’s behaviour highlighted the need for the ministerial code to be underpinned by law. The Institute for Government (IFG) report cited the decision not to dismiss Hancock, who resigned as health secretary, and said that, in light of other government revelations such as about lobbying on behalf of Greensill Capital, Boris Johnson had allowed standards of ministerial behaviour to weaken. Hancock was pictured in an embrace at his office with Gina Coladangelo, a longtime friend and a non-executive director at Hancock’s health department, on 6 May. The Labour letter, signed by Charles Falconer, the shadow attorney general, David Lammy, the shadow justice secretary, and Nick Thomas-Symonds, the shadow home secretary, notes that Hancock’s contact with Coladangelo was not permitted under social distancing rules in place at the time. “These are the rules and regulations which the vast majority of the British public made huge sacrifices to follow – including by missing birthdays, weddings and even funerals of close family members and friends,” it says. “To maintain confidence in the criminal justice system, the public need to know that the law will apply equally to everyone irrespective of rank, job, or title. The footage of the former health secretary and his aide is clear evidence that the law on indoor gatherings was breached. It appears impossible to describe the gathering shown in the footage as reasonably necessary for the purposes of work.” Citing also the lack of any criminal proceedings against Dominic Cummings when he broke Covid-related rules last year, the letter asks Ellis for “assurances from you that you have investigated the absence of any investigation in respect of Matt Hancock’s case and that you are satisfied that due process had been followed”. Ellis appears unlikely to act. When asked about the subject in parliament on Thursday by Ellie Reeves, the shadow solicitor general, Ellis said ministers “do not discuss individual cases, putative or otherwise”. He told Reeves: “I don’t recognise the problems that she raises. It is a matter for the independent authorities to investigate where they see fit and not a matter for government ministers.” The IFG report argues that as part of a review of the ministerial code, the handbook for conduct by ministers should be given a basis in law, as with parallel codes for civil servants and political special advisers. It also calls for Johnson’s independent adviser on ministers’ interests, Christopher Geidt, to be given the independent power to begin investigations and publish findings without the agreement of the prime minister. Citing also Johnson’s decision to take no action against Priti Patel after a formal investigation concluded that the home secretary had bullied civil servants, prompting the resignation of Geidt’s predecessor, Sir Alex Allan, the IFG said Johnson had “undermined the code”. Tim Durrant, an associate director at the IFG, said: “The prime minister claims to value high standards of behaviour in government, but under his watch the system upholding them has been severely weakened. His decision to publish a new version of the ministerial code is an opportunity to repair some of the damage.”