This follows amid allegations that he used his position as London mayor to get Jennifer Arcuri f, avourable treatment.
Jennifer Arcuri has reportedly admitted in a newspaper interview to having had an affair with Boris Johnson.
Arcuri told a United Kingdom local newspaper Daily Mail in an interview that she did have an affair with the prime minister, when he was mayor of London.
City Hall resumed a probe into Mr Johnson's conduct after the police watchdog closed its investigation in May.
"I think that goes without saying".
Boris Johnson and Marina Wheeler
Johnson avoided a criminal investigation after a United Kingdom police watchdog found no evidence to confirm the allegations. It's pretty much out there...
However, it found that the prime minister would have been "wise" to declare a conflict of interest and said failure to do so could have constituted a breach of the so-called Nolan principles on conduct expected from public servants, which include integrity, selflessness, openness and honesty.
The scandal surfaced after The Sunday Times reported in late September 2019 that Arcuri had allegedly told four friends and a Tory activist that she was engaged in an intimate relationship with Johnson while he served as the mayor of London and was married to his erstwhile wife, Mariana Wheeler.
Mr Johnson, who was the capital's mayor between 2008-2016, previously welcomed the IOPC finding and criticised the "vexatious claims" against him.
The reports were followed by allegations that the United Kingdom prime minister had engaged in misconduct by showing preferential treatment to Arcuri, including her in private trade talks and facilitating grants of up to £100,000 from state bodies.
IPOC director general Michael Lockwood also said in a statement: "The IOPC completed a thorough, independent and impartial assessment to determine if there were reasonable grounds to suspect the criminal offence of misconduct in public office had occurred".