Boris Johnson should not be involved in selecting the new head of the Metropolitan police while he remains under investigation for alleged lockdown breaches, a former force commissioner has said. Ian Blair, a former Met commissioner, called on the prime minister to recuse himself from involvement in the process, describing the decision about Cressida Dick’s replacement as “an enormously important choice”. Lord Blair’s remarks echo a number of calls for Johnson to stay out of the process, with the opposition warning it would entail a serious breach of ethical standards if he is involved while being investigated by the force. The prime minister was sent a legal questionnaire by Scotland Yard on Friday over the alleged parties in Downing Street, No 10 confirmed, signalling that police are investigating whether Johnson personally broke the law. He has seven days to adequately explain his attendance or be fined for breaking the rules he himself set. Discussing the process of replacing Dick after the London mayor, Sadiq Khan, made clear she had lost his confidence, Blair told BBC Radio 4’s Today programme: “That’s a very difficult issue and I do accept that. I think Boris Johnson should recuse himself from being involved. “But it’s an enormously important choice and presumably it will take some time to get to being to who the commissioner is, and by that stage presumably he will have filled in his questionnaire and the matter will be over.” Dick’s dramatic resignation on Thursday evening came hours after she told a radio phone that she had “absolutely no intention” of standing down and had a plan to rid the Met of its toxic culture. She was forced out after Khan made it clear he had no confidence in her plan to reform the force after a series of scandals revealing racism, misogyny and homophobia within elements of the force. No 10 said the prime minister had no official role in choosing the Met commissioner and the final decision would be made by the home secretary, Priti Patel, who is a close ally of Johnson. However, Brian Paddick, a former senior Met officer who is now a Liberal Democrat peer, said fairness in the appointment process could only be guaranteed by Johnson stepping down as prime minister. “It is inconceivable anyone will be appointed as commissioner of the Met without the approval of the PM,” Paddick said. “When you have a PM under criminal investigation, that is very problematic. I’m unconvinced that the PM giving his personal assurance that he will take no part will reassure the public.” The calls for Johnson to distance himself from the process come after Angela Rayner, Labour’s deputy leader, expressed alarm at a Times article that quoted an unnamed “senior ally” of the prime minister saying Scotland Yard should exercise caution when making a decision that could see Johnson forced out. “This raises very serious questions about both the rule of law and basic standards of government,” she said. “If the prime minister and his allies are going to publicly lean on the police to try to get him off the hook for an offence, he cannot then get involved in appointing a commissioner who would decide whether to charge him.” Rayner urged the government to “make clear that no one under investigation will be involved in any way”, pressuring Johnson to “say now if he will recuse himself or explain how public confidence will be maintained in a fair process”.