The police will investigate alleged parties held at Prime Minister Boris Johnson's office due to an apparent violation of Covid-19 rules.
The event was held to celebrate Johnson's birthday on June 19, 2020.
The decision to open an investigation followed evidence provided by the Cabinet Office, according to the London police chief.
I can tell you this morning that the Met is investigating a number of events that took place at Downing Street, as a result of information provided by the Cabinet Office inquiry team and my officers own assessment.
Dick said that several events that appeared to have taken place at Downing Street were not considered to have reached the threshold for a criminal investigation based on the information available.
Johnson has resisted calls to resign from across the political spectrum despite public anger over a long and growing list of alleged lockdown breeches.
Dick acknowledged that there was a deep public concern about allegations of lockdown parties in violation of the government's own guidelines.
The Met had previously been criticized for dismissing calls to look into the claims.
The results of Sue Gray's investigation into the partygate scandal were expected to be published in the coming days. Gray's report may be delayed because of the Met's investigation.
The Gray investigation's terms of reference state that if any evidence emerges that is potentially a criminal offence, the matter will be referred to the police.
The pound dipped on Tuesday morning. The British currency was at a value of $1.3449 shortly after noon. London time is near a three-week low.
The Met has sought to take a proportional approach throughout the Pandemic. Dick said that the fact that the Met is investigating doesn't mean that fixed penalty notices will be issued in every instance and to every person involved.
We will not be giving a running commentary on our current investigations, but we will give updates at significant points as we would normally do.