It follows reports Mr Johnson gave a speech at a packed leaving do for a senior aide in November last year during the second lockdown, with 40 or 50 people crammed “cheek by jowl” into a medium-sized room.
Speaking at Prime Minister’s Questions, the Labour leader insisted that any festive event would have been against the rules that applied in 2020.
He said: “As millions of people were locked down last year, was a Christmas party thrown in Downing Street for dozens of people on December 18?”
Mr Johnson told Sir Keir: “All guidance was followed completely.”
The former director of public prosecutions replied: “The defence seems to be ‘no rules were broken’.
"Well, I’ve got the rules that were in place at the time, Prime Minister, of this party. They are very clear – you must not have a work Christmas lunch or party.
“Does the Prime Minister really expect the country to believe that whilst people were banned from seeing their loved ones at Christmas this year, it was fine for him and his friends to thrown a boozy party in Downing Street?”
The Prime Minister replied: “I have said what I said about Number 10 and the events of 12 months ago.”
The Labour leader responded: “The Prime Minister does not deny there was a Downing Street Christmas party last year. He says no rules were broken.
“Both of those things can’t be true, Prime Minister. He is taking the British public for fools”.
Deputy Labour leader Angela Rayner said: “The Prime Minister has lived his life never facing consequences for breaking the rules.
“But when you occupy Number 10 during a pandemic, that’s not just wrong, it’s dangerous. We can’t have one rule for him and another for everyone else any more.”
Mr Johnson was also challenged over the party by SNP Westminster leader Ian Blackford, who claimed the Mirror newspaper have now taken legal advice on “potential illegality” over the party.
He said: “Last Christmas the Prime Minister hosted a packed party in Downing Street, an event that broke lockdown rules.
“The Prime Minister might be denying it, but I’ve spoken to the Mirror newspaper this morning and they are confirming what happened, and they have legal advice on potential illegality.
“At a time when public health messaging is so vital, how are people possibly expected to trust a Prime Minister when he thinks it’s one rule for him and one rule for everybody else?”
The Prime Minister replied: “He’s talking total nonsense. I think he would have been better off, frankly, saying something about the victims of Storm Arwen in Scotland.”
Mr Johnson said the UK Government and Scottish authorities needed to work together to get their power back “and that’s what we’re doing”.
The SNP Westminster leader labelled the Prime Minister’s answer “disgraceful”.
He said: “This is at a time when leadership matters, when truth matters and when trust really matters."