London's police chief on Tuesday said her officers are investigating several parties that took place at Prime Minister Boris Johnson's office and government departments during Covid lockdowns.

"The Met is now investigating a number of events that took place at Downing Street and Whitehall in the last two years in relation to potential breaches of Covid-19 regulations," Metropolitan Police Commissioner Cressida Dick told the London Assembly local authority.

Allegations that a string of parties were held at Downing Street while the rest of the country was in lockdown have shaken Johnson's government in recent weeks, prompting the worst crisis of his premiership and calls for him to quit.

The latest revelations came on Monday night and saw claims that Johnson broke lockdown rules by having a birthday party at Downing Street on June 19, 2020.

Up to 30 people were present, ITV News alleged. At the time, social gatherings were only permitted between six people outside.

"I should stress that the fact that we are now investigating does not of course mean that fixed penalty notices (fines) will necessarily be issued in every instance and to every person involved," Dick told the Assembly's Police and Crime Committee.

"We will not be giving a running commentary on our current investigations. But I can assure you that we will give updates at significant points as we would generally do."

A senior civil servant, Sue Gray, is also conducting an investigation into the string of allegations and is expected to publish her investigation soon.

Speculation has swirled that she would have to pause her fact-finding probe if the police become involved.

But a Cabinet Office spokesperson said: "The investigation being carried out by Sue Gray is continuing. There is ongoing contact with the Metropolitan Police Service."

Gray's investigation is understood to include the claims about the June 19, 2020 birthday party for Johnson.