A party did take place, sources have told ITV News Political Editor Robert Peston - but did that break the rules, or the spirit of the rules?
His team is accused in the paper of breaching restrictions by holding their own festive party in Downing Street days before Christmas, while the rest of London was effectively locked down under Tier 3 restrictions.
In each case, the paper reported, there were 40 or 50 people crammed "cheek by jowl" into a medium-sized room.
In response to the allegations reported in The Mirror, a Downing Street spokeswoman said: “Covid rules have been followed at all times”.
ITV News Political Editor Robert Peston says his sources have confirmed a party did happen, but whether official rules were broken (or indeed the spirit of the rules) remains unclear.
At Prime Minister's Questions, Mr Johnson was accused by Sir Keir Starmer of "taking the British public for fools" as the Labour leader insisted that any festive event would have been against the rules that applied in 2020.
Sir Keir directly asked Mr Johnson: "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."
Sir Keir said: "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?"
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The PM 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".
The Scottish National Party's leader in Westminster, Ian Blackford, asked: "How are people possibly expected to trust the Prime Minister when he thinks it's one rule for him and one rule for everybody else?"
But Mr Johnson told him he was "talking total nonsense".