On Wednesday, he urged the PM's resignation, claiming the public could see him "lying through his teeth." When asked about the "work event" in the No10 garden during the initial lockdown, Mr Johnson apologised and claimed he thought it was "a work function".
That the Conservative leader was at the purported party shows the Government is not following its own lockdown guidelines, Labour says.
Sir Keir's criticism came despite being photographed drinking with party staff last May, a senior Tory said last night.
Sir Keir was seen drinking a beer with two others.
The shot was taken outside a Durham constituency office days before the Hartlepool by-election.
During the second phase of the third lockdown, indoor mixing between homes was prohibited save for work.
The Conservative insider told The Telegraph of Corbyn: "Sir Keir has spent the last two months chastising others for doing the same thing he does.
"He is a hypocrite."
"He might explain why there are images of him quaffing beer - we haven't heard him do so," Mr Johnson said earlier this week during Prime Minister's Questions.
Labour has consistently denied breaking any regulations.
The party said: "Keir was at work, meeting a local MP and taking part in an online Labour Party event.
"They paused for dinner because the meeting was late."
According to reports, Mr Johnson had parties in Downing Street while the rest of the country was on lockdown.
The scandal has enraged Tory MPs, who worry public confidence in the party may be irreversibly harmed.
Five Conservative MPs publicly called for the PM's resignation, while dozens more privately questioned his leadership.
A senior federal servant would investigate the accusations of the gatherings, Mr Johnson said.
The investigation has interrogated ministers, special advisers, and officials to determine if the law was broken.
The conclusions of the probe are expected to be published by the end of next week.
The Met Police say they will consider opening a criminal investigation once the report's findings are revealed.