"The Metropolitan Police service is aware of widespread reporting relating to alleged breaches of the Health Protection Regulations at Downing Street on 20 May 2020 and is in contact with the Cabinet Office," the force said in a statement on Monday night.
The statement was issued after an email shared exclusively with ITV News provided the first evidence of a party on that date, when the rest of the country was banned from meeting more than one other person outdoors.
Downing Street staff were invited to a drinks party in the Number 10 garden during the height of nationwide lockdown to "make the most of the lovely weather".
The email was sent by the Prime Minister's Principal Private Secretary Martin Reynolds to over a hundred employees in Number 10, including the Prime Minister's advisors, speechwriters and door staff. In it, Mr Reynolds - a senior No 10 civil servant who has run Boris Johnson's private office since October 2019 - says: "Hi all, "After what has been an incredibly busy period we thought it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No10 garden this evening. "Please join us from 6pm and bring your own booze!"
Watch ITV News UK Editor Paul Brand's report on the email which proves Downing Street staff held drinks party at height of lockdown
ITV News understands around 40 staff gathered in the garden that evening, eating picnic food and drinking.
Crucially, they included the Prime Minister and his wife Carrie Johnson.
When Mr Johnson was asked on Monday if he and his wife attended the party on May 20, he replied: “All that, as you know, is the subject of a proper investigation by Sue Gray."
Downing Street told ITV News they would not comment on the story due to the Sue Gray inquiry.
The cabinet office inquiry into allegations of various parties and events is currently being carried out by senior civil servant Sue Gray, who took over from the Cabinet Secretary Simon Case due to allegations that he knew of at least one party himself.
Who is the 'we' in the invite and have police been left with 'no option' but to investigate? UK Editor Paul Brand and Political Editor Robert Peston explain
Responding to our report, Labour's Deputy Leader Angela Rayner said: "It's terrible and I think many people that see the evidence now will not, only think that Boris Johnson's lies are catching up with him, but will see it as absolutely despicable that when they were told to follow the rules, Boris Johnson and Number 10 were breaking the rules." "It's disgraceful and he should be ashamed," Ms Rayner added.