However, he insisted his staff had repeatedly assured him “there was no party and that no Covid rules were broken”.
Mr Johnson told Parliament he has asked Cabinet Secretary Simon Case “to establish all the facts and to report back as soon as possible”
He added if rules were broken “then there will be disciplinary action for all those involved”.
During Prime Minister’s Questions, he said: “I understand and share the anger up and down the country at seeing No 10 staff seeming to make light of lockdown measures, and I can understand how infuriating it must be to think that people who have been setting the rules have not been following the rules because I was also furious to see that clip.”
The Labour leader said: “The British people put the health of others above themselves and followed the rules. Isn’t the Prime Minister ashamed that his Downing Street couldn’t do the same?”
The Prime Minister said the events on December 18 “will be properly investigated”.
He added: “What people should not do is lose focus on what we are trying to do now.
“Of course we will deal with the events of what may or may not have taken place on December 18 last year, but what I think we need to focus on today is what we’re doing to roll out the vaccinations across this country and what we’re doing to protect the public.”