Witnesses said the PM and his wife were among 30 persons present on May 20, 2020.
But Boris Johnson has refused to acknowledge if he was there.
The Met said it was in touch with the government about "widespread reporting" of alleged Covid regulations violations.
Socially distant drinks in the No. 10 garden tonight, according to an email obtained by ITV News.
It was sent while the lockdown was in effect. On June 1, England loosened its laws to enable groups of up to six people to assemble outside.
Health Minister Edward Argar told BBC Breakfast he understood "many's indignation, anguish, and upset".
But he added it was "essential" to wait for the results of a probe into Downing Street gatherings.
If proven to have attended, the PM would face "severe questions."
Ed Miliband said Mr Johnson would be in a "very difficult situation" if found to have "violated the rules and maybe the law."
"He has to come straight, explain himself, not hide behind investigations," he remarked.
On Monday, Downing Street declined to comment, citing an ongoing independent probe by senior civil servant Sue Gray into the gathering.
A Met spokesperson said the agency was "aware of widespread complaints pertaining to alleged breaches of the Health Protection Regulations at Downing Street on 20 May 2020 and is in contact with the Cabinet Office."
The May 20th gathering was revealed last week in a blog by former PM adviser Dominic Cummings.
ITV News has now revealed what appears to be the entire email sent on Mr Johnson's behalf by Martin Reynolds.
"Socially Distanced Drinks! [OFFICIAL-SENSITIVE-NO 10 ONLY]" it says.
To celebrate the great weather, the email invites guests to "socially distanced beverages" in the No. 10 garden.
"Come at 6pm and bring your own booze."
Two witnesses told the BBC they spotted Mr Johnson and Carrie during the garden party.
"There were conversations about the email being incredibly unwise," one insider claimed.
"People said, 'What the hell?'" said another.
The gathering was believed to be held at a long table in the garden.
Messages sent between Downing Street staffers at the time were shown to the BBC "Oh. Why does Martin (Reynolds) encourage a garden party?" "Is this real?" said another.
The Metropolitan Police advised individuals that they could only rest or exercise outside alone, with their own household or one person from another household, on the day of the gathering.
On the same day, the then Culture Secretary, Oliver Dowden, issued the Downing Street coronavirus update, confirming 363 more Covid deaths in the UK in the previous 24 hours.
"You can meet one person outside your household in a public location if you stay two metres away," he urged the crowd.