The results of Sue Gray's probe into allegations of lockdown-flouting parties look set to be delayed while Scotland Yard carry out an investigation.
Metropolitan Police Commissioner Dame Cressida Dick confirmed the force would look at "a number of events" in Downing Street and Whitehall during the pandemic.
Dame Cressida told the London Assembly today: "What I can tell you this morning is that as a result of the information provided by the Cabinet Office inquiry team and, secondly, my officers' own assessment, I can confirm that 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."
The police have been under mounting pressure to launch an investigation into the claims - with the Mirror first reporting allegations of rule-breaking gatherings back in November.
Whitehall enforcer Sue Gray had been expected to publish the results of her inquiry into the 'partygate' saga this week - with Boris Johnson's future hanging in the balance.
But the Mirror understands that the report will not be published in full until the Met Police finish their investigation.
Downing Street said Ms Gray will halt her work on any party allegations being looked at by the police.
But she is free to publish findings on other parts of her inquiry that police are not considering, No 10 suggested
The PM's official spokesman said: "As the terms of reference make clear, they won't publish anything that relates to the work of the police, there are a number of events and allegations that they have looked into that the police said don't reach their threshold, which they are able to continue looking into.
"And it is my understanding that they will be able to publish detail about those events rather than ones which the police might be taking forward."
He added: "It's up to the investigation team when they publish, it's my understanding they are able to publish the aspects that aren't a matter for the police."
The spokesman was unsure whether the Gray inquiry would return to other events once the police investigation concludes.
It is unclear how long it will take the police to carry out their investigation.
Dame Cressida declined to say how many events were being looked at, only that the police had several in their sights.
"The fact that we are now investigating does not, of course, mean that fixed penalty notices will necessarily be issued in every instance and to every person involved," she said.
A Cabinet Office spokesman said: "The investigation being carried out by Sue Gray is continuing.
"There is ongoing contact with the Metropolitan Police Service."
Deputy Labour leader Angela Rayner questioned how Mr Johnson can remain Prime Minister while No 10 is subject to a police investigation.
"Boris Johnson is a national distraction," she said.
"Conservative MPs should stop propping him up and he should finally do the decent thing and resign."
Families who lost loved ones during the pandemic said the report was a "shameless attempt to save face" - and called on the PM to resign.
Matt Fowler, co-founder of Covid-19 Bereaved Families for Justice, said: "Every day Boris Johnson remains in office is a disaster for the country’s pandemic response, that unnecessarily puts more lives at risk.
"Nobody outside of Westminster thinks we need to wait for Sue Gray's report. The Prime Minister has already lost all credibility and the fact that the leader of the country is now under police investigation is deeply reprehensible . He needs to resign.
"The Sue Gray “inquiry” is clearly a shameless attempt to save face. The only inquiry with any credibility is the independent one led by Baroness Hallet, which is about learning lessons that will save lives, and give those of us that have lost loved ones the answers we deserve."
