The PC interrupted the bash attended by around 50 people on December 18, 2020, but it turned out not to be an emergency. The officer is a key witness in the probe into allegations that Downing Street personnel blatantly flouted lockdown rules while the rest of the country obeyed the law.
He has given evidence to Sue Gray, the senior civil servant probing the allegations, and will be interviewed by detectives investigating alleged breaches of the pandemic legislation.
Metropolitan Police Deputy Commissioner Sir Stephen House revealed details of the incident at a meeting of the London Assembly’s police and crime committee.
Sir Stephen said: “One officer was involved in responding to a silent alarm we believe had been pressed in error. That officer has been spoken to by the Sue Gray investigation and will be spoken to by ourselves.”
The Deputy Commissioner was asked why police on duty at Downing Street did not intervene and break up the allegedly illegal parties.
Sir Stephen said officers, most of them armed, were there to prevent terrorist attacks.
He rejected suggestions they turned a blind eye to breaches. He added: “Armed officers do not routinely walk through the Cabinet Office or No10 Downing Street.
“Many of the events we’re investigating were at the end of the working day for people already in the building and who had been there since 7am or 8am.”
But Sir Stephen added: “If we find officers knew what was going on and should have intervened, then we will follow up on that.”
The Met has sent questionnaires to 88 people who attended allegedly lockdown breaching parties – including the PM.
Questions relate to eight gatherings between May 20, 2020, and April 16, 2021.