A police officer who responded to a silent alarm accidentally being triggered at Number 10 has been spoken to as part of the partygate investigations.

Metropolitan Police Deputy Commissioner Sir Stephen House said that the officer and a civilian employee responded to the alarm being sounded in error at Downing Street.

They have been spoken to as part of the Sue Gray investigation into alleged parties at Downing Street and the Cabinet Office during lockdown, and will also be spoken to by police.

Sir Stephen said: “One officer was involved in responding with a civilian custodian, not one of our employees, a civilian custodian who works for Number 10, with responding to a silent alarm, which we believe had been pressed in error, and that officer has been spoken to both by the Sue Gray investigation and will be spoken to by ourselves.”

Asked why officers guarding the site had not taken action to stop allegedly lockdown-busting parties, he told members of the London Assembly Police and Crime Committee that they are there to prevent terrorist attacks.

He said: “Armed officers do not routinely … walk through the Cabinet Office or Number 10 Downing Street.”

But Sir Stephen added: “If we find officers knew what was going on and should have intervened, then we will follow up on that.”

Investigators have already received answers to some of the more than 50 questionnaires sent out to those believed to have attended the events.

He told the committee: “We are starting to receive questionnaires back and they are now being reviewed as to whether or not there is a reasonable excuse within that response.

“That means that the person did not breach Covid regulations.

“If there is not what we believe to be a reasonable excuse in there then we will go ahead to move towards issuing of fixed penalty notices.”