Scotland Yard is not prosecuting former London mayoral candidate Shaun Bailey or anyone else who attended a gathering at Conservative Party headquarters during a Covid lockdown.
While London was under restrictions in December 2020, Mr Bailey and Tory aides were seen posing for a photo, raising glasses beside buffet food.
Indoor socialising was prohibited.
However, the Met stated that there was “insufficient evidence to disprove the version of events provided by attendees.”
Fresh questions about the event at Conservative Campaign Headquarters in Westminster were raised after it was reported that former prime minister Boris Johnson had put Mr Bailey forward for a peerage in his resignation honours list.
After the Daily Mirror published a picture of the gathering in December 2021, the Metropolitan Police launched an investigation.
Then-Transport Secretary, Grant Shapps, who is now business secretary, said “that scene is absolutely unacceptable”.
Before the photo was published, the Times had reported that a “raucous” party took place in the basement while Covid restrictions were in place.
The Met concluded the “photo by itself is not sufficient evidence on which to assess that an offence had been committed”.
Officers issued attendees with questionnaires to decide whether breaches of the rules were committed and fines should be issued.
“The investigation reviewed all the material thoroughly and after careful consideration, it was determined that there was insufficient evidence to disprove the version of events provided by attendees to a standard that would meet the threshold required,” the Met said.
“As a result, a decision was made that no further action should be taken.”
Mr Bailey, who remains a London Assembly Member, resigned as chairman of the governing body’s Police and Crime Committee after the picture emerged.
Scotland Yard issued a total of 126 fines over rule breaches in Whitehall and Downing Street while Mr Johnson was prime minister, in a scandal that helped end his tenure in No 10.
Mr Johnson and his then-chancellor Rishi Sunak paid fixed-penalty notices over a gathering held for Mr Johnson’s 56th birthday