As many as 100 people were invited to a “bring-your-own-booze” drinks event in the Downing Street garden during the first lockdown, it is understood.
Witnesses have told the BBC the PM and his wife were among about 30 people who attended the gathering on 20 May 2020.
But Boris Johnson has declined to say whether he was among those there.
The Metropolitan Police said it was in contact with the government over “widespread reporting relating to alleged breaches” of Covid rules.
Labour has been granted an urgent question in the Commons at 12:30 GMT to push for answers from a minister.
The party’s leader, Sir Keir Starmer, said the prime minister needed to “come clean” about the gathering.
Addressing Mr Johnson on Twitter, he added: “Your deflections and distractions are absurd. Not only did you know about the parties in Downing Street, you attended them.
“Stop lying to the British public.”
An email, revealed by ITV News, invited people to “socially distanced drinks in the No 10 garden this evening”.
It was sent when lockdown restrictions were still in place. Twelve days later – on 1 June – rules in England were relaxed to allow groups of up to six people to meet outside.
Health Minister Edward Argar told BBC Breakfast he understood the “anger, sadness and upset many will feel about these allegations”.
But he said it was “important” to allow an investigation to take place into gatherings in Downing Street before coming to any conclusions.
However, former Scottish Conservative leader Ruth Davidson told the BBC the PM did not need to wait for an inquiry to answer whether he attended.
“I can’t understand if the prime minister attended why on earth he thought that was the right thing to do,” she added.
“People are furious about this, what they gave up, how difficult it was, what happened to their own lives, and to think that this was going on? It is indefensible, it cannot be defended.”
On Monday, Downing Street declined to comment as it said the gathering – like others reported to have happened under coronavirus restrictions during 2020 – is now being looked into as part of an ongoing independent investigation by senior civil servant Sue Gray.
The Liberal Democrats have called for a police investigation, and a Met spokesperson said the force was “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”, which oversees government policy.
Shadow climate change secretary Ed Miliband told BBC Radio 4’s Today programme the prime minister “cannot run and cannot hide” from the allegations.
He added: “It speaks to a rotten culture at the heart of this government and the rotten culture begins with the person in charge.”
IMAGE SOURCE,PA MEDIAImage caption,Hannah Brady said her father’s death certificate had been signed on the same day the drinks took place
The Wednesday 20 May gathering first came to light last week in a blog by the PM’s former adviser Dominic Cummings.
ITV News has now published what appears to be the full email invitation sent on behalf of Mr Johnson’s principal private secretary, Martin Reynolds.
It has the subject line “Socially Distanced Drinks! [OFFICIAL-SENSITIVE-No 10 ONLY]”.
“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 No 10 garden this evening,” the email says.
“Please join us from 6pm and bring your own booze.”
Two eyewitnesses have told the BBC they saw Mr Johnson and his wife Carrie at the event in the garden.
And sources have said that several Downing Street members of staff voiced concerns about drinks happening, with one claiming: “There were conversations about the email being really unwise.”
Another added: “I remember people saying, ‘What the hell?'”
Others said a long table had been set up in the garden for the event.
A message sent by one Downing Street staffer to another at the time, shown to the BBC, said: “Um. Why is Martin (Reynolds) encouraging a mass gathering in the garden?” Another asked: “Is this for real?”
In a Twitter post on the day of the gathering, when it was 25C (77F) in London, the Metropolitan Police reminded people they could only relax or exercise outside on their own, with their own household or one person from another household.
On the same day as the gathering, the then Culture Secretary, Oliver Dowden, gave the Downing Street coronavirus briefing – in which he confirmed another 363 people in the UK had died with Covid in the previous 24 hours.
At the briefing, he told the public: “You can meet one person outside of your household in an outdoor, public place provided that you stay two metres apart.”
BBC
