Mr Bailey, who is believed to be in line for a peerage, and Tory aides were seen posing for a photo while raising glasses beside buffet food while London was under coronavirus rules.
Several of them wore festive hats, and one was in a House of Commons Christmas jumper as they smiled for the camera while indoor socialising was banned in December 2020.
But the Metropolitan Police said in a statement on Friday that officers found “insufficient evidence to disprove the version of events provided by attendees”.
Questions over the event at CCHQ in Westminster were raised after it emerged that Boris Johnson has put Mr Bailey forward for a peerage in his resignation honours list.
The former Conservative candidate to be London’s mayor apologised “unreservedly” for the event organised by his campaign team and said it was a “serious error of judgment”.
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 Times reported that a “raucous” party took place in the basement while Tier 2 restrictions were in place.
After the Daily Mirror published the picture at the gathering in December last year, the Met launched an investigation.
But the force concluded that the “photo by itself is not sufficient evidence on which to assess that an offence had been committed”.
Officers issued attendees of the event on December 14 2020 with questionnaires to determine 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.
The Met issued 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.