There are now calls for Cabinet Secretary Simon Case to be removed from the inquiry.
The Cabinet Office held two Christmas parties in December 2020, according to the Guido Fawkes website.
This occurred during a time of societal limits and challenges.
Around 15 people were invited to one event, which included drinking and music.
A source told the site that Mr Case did not attend, but did ‘stick his head into the room to call workers out' during the celebration.
One of the events was on December 17, the day before the infamous Downing Street Christmas party, the Times says.
The event was coordinated by Mr Case's personal secretary and labelled as ‘Christmas party!' on digital calendars.
An investigation by Politico and The Independent found Mr Case enjoyed drinks with 15 to 20 colleagues at his office and in the Cabinet Office waiting area.
Mr Case should be removed from the investigation and resign, says SNP Westminster leader Ian Blackford.
Mr Johnson and Tory chairman Oliver Dowden both expressed confidence in Mr Case's findings.
Tories lost North Shropshire by-election due to media reportage of lockdown-busting parties, according to Mr Johnson.
People have been hearing a steady barrage of stuff about politics and politicians, he said.
While thousands were forced to spend Christmas apart due of mandatory coronavirus restrictions, Mr Blackford said: He and his report must go.'
In a letter to the PM, he recommended it should be judge-led.
To put it simply, the public suspected a rigged, limited internal investigation and a cover up. They now know your investigator was also engaged.'
Deputy Labour leader Angela Rayner said: ‘Each revelation shows a culture of ignoring the rules.
A person directing the investigation should be uncorrupted and able to make an independent judgement. That is now in dispute.'
‘These accusations are totally untrue,' the Cabinet Office informed Politico and The Independent.
We've contacted the Cabinet Office.