The Met Police will wait for the result of the civil service probe looking into the No 10 party in May 2020, attended by Boris Johnson, before deciding whether to investigate.
They said they would give it “further consideration” if civil servant Sue Gray’s inquiry into parties unearthed potential criminal offences.
The force said they rarely investigated breaches of Covid law retrospectively.
But Lib Dem leader Ed Davey said the police “mustn’t turn a blind eye”.
The PM is facing fury from his own party over the drinks on 20 May 2020.
He admitted at Prime Minister’s Questions on Wednesday that he had joined colleagues in the Downing Street garden for around 25 minutes to thank them for their hard work during the pandemic.
But Mr Johnson claimed he had “believed implicitly that this was a work event”.
Four senior Tories have publicly called for his resignation since the statement, including the leader of the Scottish Conservatives, Douglas Ross.
But his cabinet has rallied around him, with Culture Secretary Nadine Dorries the latest to offer support to her boss.
Ministers have appealed to angry MPs to wait for the outcome of an inquiry into all the allegations of lockdown parties across government – led by senior civil servant Sue Gray – before making any firm conclusions on the PM.
In a statement released on Thursday, the Met said it was “aware of widespread reporting relating to alleged breaches of the Health Protection Regulations” in Downing Street.
The force said its “four Es” approach – mirrored by officers across the country – saw them try to “engage”, “explain”, and “encourage” people regarding the law, before having to resort to “enforcement”.
And it said officers “do not normally investigate breaches of Coronavirus regulations when they are reported long after they are said to have taken place”.
But the Met added: “If significant evidence suggesting a breach of the regulations becomes available, officers may review and consider it.”
The force said it had “ongoing contact with the Cabinet Office” over Ms Gray’s inquiry, adding: “If the inquiry identifies evidence of behaviour that is potentially a criminal offence it will be passed to the Met for further consideration.”
Lib Dem leader Sir Ed said the evidence that the prime minister broke the law was clear.
“Cressida Dick [the Met chief] mustn’t let him off the hook through a shady establishment stitch-up. The police don’t need the government’s permission to investigate a crime, and they mustn’t turn a blind eye to criminality just because it is committed by Boris Johnson.”It is ludicrous to pretend that we can leave it to a civil servant appointed by Boris Johnson to get to the bottom of this.”