The acting Metropolitan Police Commissioner has indicated that Boris Johnson escaped a fine for attending his former director of communications’ leaving party because police considered it a work event.
Sir Stephen House said he had personally been involved in decisions over the partygate investigation and did not believe the Prime Minister breached the law numerous times.
Appearing before the London Assembly police and crime committee, Sir Stephen insisted he was confident of the integrity of the investigation.
Scotland Yard came under intense pressure to explain why Mr Johnson was not given a fixed penalty notice when photographs emerged showing him raising a toast at a gathering to mark the departure of Lee Cain, his former director of communications, on Nov 13 2020.
The Metropolitan Police was also accused of lacking consistency over its decision to fine Mr Johnson for attending his surprise birthday party, but not for the five other gatherings he was at.
At the time of Mr Cain's leaving party, the country was in the grip of a second national lockdown - with the law "prohibiting indoor gatherings of two or more people except for permitted exemptions including where the gathering is reasonably necessary for work purposes".
Sir Stephen said: "I accept that many of the photographs we are seeing look bad and Sue Gray's report has dealt with that. We deal with the law, not what looks bad."
Sir Stephen said it had been a "complicated" and "detailed" investigation, with detectives examining hundreds of emails, electronic door logs, diary entries, witness statements, photographs, CCTV images and questionnaires.
He said the presence of alcohol did not necessarily mean there had been a breach of the regulations.
He told the London Assembly: "I was involved in these decision-makings myself. I am very confident of the integrity of the decisions that were made in this investigation, which was a difficult investigation."
Explaining why some people had been fined and others had not, he said: "We looked at whether there was a prescribed exemption under the regulations for the gathering, including whether it was reasonably necessary for work purposes.
"Some gatherings we decided were not work-related and some we decided were work-related."
While refusing to elaborate on specific cases, Sir Stephen's explanation appeared to confirm that the Metropolitan Police had concluded the birthday party in June 2020 was not a work event, but the other gatherings - including the Lee Cain leaving party - were.
Sir Stephen told the committee the length of time someone had spent at the gathering was also a critical factor in deciding whether to issue a fixed penalty notice, suggesting those who left early were more likely to escape.
He explained: "We also included consideration of the nature of the gathering, different phases of the gathering and the amount of time spent there by any participant."
Sir Stephen - who took over as acting Commissioner in April, when Dame Cressida Dick stepped down - also stressed that fines were only issued in cases where the force believed it had the evidence to win a case at court.
He went on: "I believe that the decisions my officers made were based on the facts and were proper.”
He said officers consulted the Crown Prosecutions Service on “generalities” but “never in relation to a specific incident".
Asked if the Metropolitan Police had treated the Prime Minister more favourably in order to avoid him being forced to resign, Sir Stephen said: "I’m not that worried what politicians think of my views. I’m not particularly concerned about what the Prime Minister thinks. I do my job without fear or favour, as did the Met in this situation."
Following Sir Stephen's appearance, Unmesh Desai, Labour’s London Assembly policing and crime spokesman, said: “In a number of cases, the Commissioner’s responses were unsatisfactory and raised more questions than answers - especially regarding the criteria for issuing FPNs in relation to an event which the Prime Minister attended on Nov 13 2020."