Boris Johnson acted with “honesty and integrity” as London mayor despite a US businesswoman claiming they had a sexual relationship while he was married, the prime minister’s press secretary has said. Allegra Stratton insisted Johnson had “no case to answer” in light of reports that City Hall has expanded its inquiry into his behaviour after Jennifer Arcuri made a series of lurid allegations. In an interview with the Mirror over the weekend, Arcuri said she slept with Johnson in his family home while his now ex-wife Marina Wheeler was away. The pair’s relationship has already come under careful scrutiny, given that Arcuri benefited from thousands of pounds in public money, including from the mayor’s promotional agency, London and Partners (L&P), and was given coveted places on trade missions to New York and Tel Aviv alongside Johnson, despite failing to meet the criteria for those trips. Johnson has never publicly accepted that he had an affair with Arcuri, but has not denied it either. When the allegations first emerged in autumn 2019, the Independent Office for Police Conduct (IOPC) investigated because Johnson, as mayor, had been head of the Mayor’s Office for Policing and Crime. It concluded that no criminal inquiry was required, but its 112-page report did say Johnson should have declared an interest in Arcuri. That was already the subject of an inquiry by the London Assembly, but Arcuri’s latest testimony is likely to fuel more questions about Johnson’s behaviour and whether – despite the fact that he may not have broken the law – his behaviour was still in line with the seven standards of public life known as the “Nolan principles”. These include that public office holders should be truthful, declare any relationships that might impugn their integrity, and “exhibit these principles in their own behaviour”. Stratton on Monday insisted that Johnson had acted correctly, telling journalists: “Of course, the prime minister follows the Nolan principle when conducting himself in public life.” Asked whether Johnson would give evidence to the London Assembly’s inquiry, she refused to confirm that he would, saying: “I’m not going to get into these hypotheticals” and, “Let’s cross those bridges when they come.” Pressed again as to whether he would cooperate, she said: “I do think the prime minister is, right now, spending all the time he can on promoting vaccine uptake, ensuring vaccine supply. All of your readers will be pleased that that is what is dominating his working capacity at the moment. It is up to the assembly what they want to do.” She repeatedly referred to the IOPC investigation, saying: “Public time, money and effort has been spent looking into whether or not there’s any wrongdoing, and it found the PM, the then London mayor has no case to answer … This has been looked at already by an independent authority.” Stratton added of Johnson: “He does believe in the wider principles of integrity and honesty. He acts with integrity and is honest. He follows the Nolan principles when conducting himself in public life.” Stratton is still not heading up the televised lobby briefings she was hired to present, and is not expected to take up the role until mid-May, after the upcoming local elections. But in a bid to avoid anger that the £2.6m taxpayer-funded studio erected in No 9 Downing Street has still not been used, Johnson himself presented the coronavirus press conference there on Monday evening.