Carrie Johnson has admitted breaking Covid rules after a photograph emerged of her hugging a friend at an engagement party.
The Sunday Telegraph published a photograph of Mrs Johnson embracing a friend at a social event at The Conduit, a private members' club in Covent Garden in September 2020.
Mrs Johnson, whose arm is wrapped around Anna Pinder in a picture that circulated online, appears to laugh with her friend as they pose for the snap on a sofa on the club's outdoor terrace.
At the time, Government guidance called for people to "stay two metres apart from people you do not live with where possible", or one metre if taking the extra precaution of meeting outdoors.
The rule of six was in place at the time the photograph was taken, which the paper said was September 17.
A spokeswoman said: "Mrs Johnson was one of a group of six seated outside celebrating a friend's engagement.
"Mrs Johnson regrets the momentary lapse in judgement in briefly hugging her friend for a photograph."
Reports have previously suggested that Mrs Johnson, a former Tory adviser, was present at the May 20 2020 "bring your own booze" event, while a photograph published in The Guardian shows her sat with her husband and Mr Reynolds in the No 10 garden on May 15, five days earlier.
The continuous revelations about several parties held in and around Downing Street at times of strict Covid measures have piled the pressure on Boris Johnson.
Six Tory MPs have called for him to resign but he is expected to try and see off the furore.
Members of Johnson's Cabinet have rallied around the PM, although some with less enthusiasm than others.
Defenders of the Prime Minister have attempted to brush off questions about all the parties - dubbed 'partygate' - and said that people should await Sue Gray's report.
In related news, Labour leader Sir Keir Starmer, boosted by polling which put his party 10 points in front of the Tories, has pledged to voters that he would preside over a government with "integrity" and that could "restore trust" if he wins the next election.
Sir Keir, writing in the Sunday Mirror, looked to keep the pressure on Mr Johnson, saying: "The Prime Minister's actions, and his inability to take any responsibility for them, means he can no longer do his basic duties.
"In the public interest, he should resign."
A survey by Opinium put Labour on 41% of the vote share, with the Conservatives on 31%. The polling company said Mr Johnson's approval rating had plummeted to minus 42% - almost a 20 point fall in a week - and that almost two-thirds of people surveyed thought he should stand down.