An alarm was accidentally set off in Downing Street on the night of an alleged Christmas party last year, ITV News has been told.
Sources inside Number 10 have confirmed that the alert was triggered around the same time as staff are said to have gathered to drink wine, eat cheese and receive jokey awards on December 18, 2020.
The revelation raises questions about how much security staff and police knew about the event at the time.
ITV News understands that the alarm was set off in a separate room to the party, where staff were at their desks working late.
One Downing Street source said that it triggered an automatic response from a custodian - or doorkeeper - who came to check whether there had been any security breach.
ITV News has also been told that a Metropolitan Police officer that night entered the reception of No 10 to make their own checks.
Senior Downing Street staff joked about holding the alleged Christmas party in question in footage obtained by ITV News
The party - attended by up to 50 people - is alleged to have taken place in a room just a few metres from the main foyer of No 10, and may have been audible from the entranceway.
Neither the Metropolitan Police nor Downing Street denied the story, with both saying they would not comment on security measures - but ITV News has spoken to several sources who have provided the same account.
So far the police have declined to investigate any of the alleged parties last year, stating that there is insufficient evidence and that they do not routinely look into breaches of Covid rules retrospectively.
But these latest revelations raise questions about whether there may have been evidence of a party at the time and whether security staff or police had any suspicions on the night.
A government spokesperson said: "Given there is an ongoing review, it would be inappropriate to comment while that is ongoing. We do not discuss matters of security.”
The Met Police said in a statement: "As a matter of course, we do not discuss specific details of protective security arrangements we provide at government buildings, as to do so could serve to undermine any such arrangements in place."
The force said they will not be investigating but could do so if the government passes them any potential evidence which comes out of the internal investigation, at which point the Met could decide to investigate after all.