Downing Street has strongly rejected reports that the prime minister broke lockdown rules last Christmas.
The prime minister's official spokesman refused to confirm whether the PM and his wife were joined by a friend, Nimco Ali, in No 10 on 25 December.
He said Boris Johnson and his then fiancee, Carrie, "had followed coronavirus rules at all times".
Residents in London were only allowed to spend Christmas Day in a support bubble or with household members.
The prime minister's spokesman faced questions from journalists after Harper's magazine claimed that Ms Ali - a close friend of Mrs Johnson - had spent Christmas with the couple at No 10.
He said Downing Street would not engage with questions about "individuals that the PM had over Christmas".
The spokesman referred back to rules put in place for families over Christmas in 2020, saying that households could "continue to use a childcare bubble" and insisted that "the PM and Mrs Johnson adhered to that at all times".
Writing on Twitter, Ms Ali also said she "did not break any rules".
Ms Ali is an independent adviser to the government on tackling violence against women and girls. She has been a close friend of Mrs Johnson since 2017 - they met when Mrs Johnson was running Lord Goldsmith's campaign to become MP for Richmond Park.
Plans to ease Covid restrictions for the 2020 Christmas period were cancelled as the infection rate increased, and Tier Four restrictions were introduced for London and much of south-east England.
Residents were asked to stay at home and were not allowed to travel to see family and friends.
Exceptions were allowed for people with young children.
Asked about reports that Mrs Johnson's mother had also visited No 10, the spokesman said the prime minister's mother and Mrs Johnson's mother "were not there over the Christmas period".