Accusing the PM of "deflections and distractions", the Labour leader in a tweet on Tuesday called on Mr Johnson to be honest with the British public about a "bring your own booze" event which was reportedly held on 20 May 2020.

Sky News understands that both Mr Johnson and his then fiancée were present at the gathering.

"Boris Johnson, 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. It's time to finally come clean," Sir Keir said.

Live COVID updates as Labour accuses government of 'ignoring the rules put in place for the rest of us'

Meanwhile, a very senior Conservative told Sky News' political editor Beth Rigby the situation is "as bad as it gets".

"The fact that [Oliver] Dowden was telling people what they couldn't do from one room then less than an hour later this was happening in the garden is indefensible," they said.

Earlier on Tuesday, Health minister Ed Argar told Kay Burley he "can understand why people are angry" about the new allegations.

Mr Argar said it is "absolutely right" that Sue Gray - the senior civil servant leading an inquiry into multiple allegations of COVID rule-breaking in Downing Street and other government buildings - looks into the matter.

He added: "I don't know what did or didn't happen."

Downing Street garden party allegations

Asked several times whether if 40 people - including the PM and his wife - had gathered in the Downing Street garden it would have been against the government's own coronavirus rules, Mr Argar did not explicitly answer the question.

In spring 2020, when the event allegedly took place, outdoor group gatherings were banned in England.

Mr Argar told Sky News it is "not appropriate" to comment on what Sue Gray "may or may not conclude" in her investigation of the Downing Street party allegations, but that if wrongdoing is found, "appropriate disciplinary action" should follow.

Hannah Brady, a spokeswoman for COVID-19 Bereaved Families for Justice, said it made her "sick" to think No 10 staff "partied" in the days after her father's death.

Put to him that the families of those who have died during the coronavirus pandemic are furious at the latest revelations in the Number 10 "partygate" row, the health minister told Sky News: "We've lost 150,230 people to this disease, so I can understand with these allegations that people will be upset and angry".

Police 'in contact with Cabinet Office'

The Met Police have confirmed they are "in contact with the Cabinet Office" following the reports of the Downing Street garden drinks party.

A spokesman for Scotland Yard said: "The Metropolitan Police Service is aware of widespread reporting relating to alleged breaches of the Health Protection Regulations at Downing Street on May 20 2020 and is in contact with the Cabinet Office."

Analysis: PM can run but he can't hide over 'partygate' and he is under enormous pressure

Mr Argar told Kay Burley that "what actually happened or didn't happen in Downing Street is a matter for Ms Gray", adding that he would not "pre-judge" her inquiry.

"[Ms Gray] will come up with her conclusions having interviewed the relevant people, having looked at the evidence and she will then conclude a) what did or didn't happen, and then b) whether anything that did happen was consistent with the rules."

Asked if he would have accepted an invitation to the alleged Downing Street drinks party during the first lockdown, Mr Argar said he spent last May "glued to my Zoom screen" but that he might have had a "quiet word" with neighbours if they were having a party.

Responding to Mr Argar's comments, former Scottish Conservative leader Ruth Davidson condemned the alleged Downing Street drinks party.

She tweeted: "This line won't survive 48 hrs. Nobody needs an official to tell them if they were at a boozy shindig in their own garden. People are (rightly) furious. They sacrificed so much - visiting sick or grieving relatives, funerals. What ** were any of these people thinking?"

Explainer: What parties are alleged to have taken place in government during lockdown?

PM dodges question on 20 May 2020 event

Earlier on Monday, the prime minister refused to answer a question about whether he and his wife attended a Downing Street party on 20 May 2020.

"All of that, as you know, is the subject of a proper investigation by Sue Gray," Mr Johnson said.

A Whitehall investigation into multiple allegations of coronavirus rule-breaking in both Number 10 and other government buildings has since been widened to include two alleged events on 15 and 20 May 2020, the latter of which was initially revealed by Dominic Cummings - the prime minister's estranged former chief adviser.

A leaked email has now emerged from Martin Reynolds - a key aide to the PM - inviting staff to "make the most of the lovely weather" with "some socially distanced drinks".

According to ITV News, who obtained the email, Mr Reynolds' message on 20 May was sent to more than 100 employees in Number 10, including the prime minister's advisers, speechwriters and door staff.

Around 40 attendees are reported to have gathered in the garden that evening to eat picnic food and drink.

Mr Cummings has previously claimed, in a blog post last week, that a photograph of himself, the prime minister, Mrs Johnson with the couple's newborn baby, and Mr Reynolds from 15 May 2020 - the date of another alleged Number 10 social event - was not an example of rule-breaking but was instead the continuation of a Number 10 meeting.

The image, published by the Guardian last month, appears to show the group enjoying wine and cheese as they sit at a table, while other groups are pictured also seemingly enjoying drinks in the Downing Street garden.

However, Mr Cummings also suggested in his blog post that a 20 May event was against COVID rules and should be investigated.

Gatherings outside were banned

At the time of the alleged 20 May event, Britons had only recently been allowed to meet with one person from another household outside as long as they remained two metres apart.

Mixing with multiple households was still banned and groups of up to six people were only allowed to meet outdoors from 1 June.

On 20 May 2020, 363 more people were announced to have died after testing positive for coronavirus.

Oliver Dowden, then culture secretary, used a Downing Street news conference at 5pm that day - an hour before the planned Number 10 drinks event - to tell the public that "the vast majority of people" should be "staying at home as much as possible".

On the same day, the Metropolitan Police posted a tweet to remind people they could only enjoy being outside if they were on their own, with people they live with, or on their own with one other person from a different household.

PM 'should be ashamed'

Deputy Labour leader Angela Rayner said Mr Johnson "should be ashamed" following further claims of rule-breaking at Number 10, which she described as "despicable".

And shadow climate change secretary Ed Miliband told Sky News there is a "pattern" of the PM "not telling the truth", adding: "The PM cannot hide behind this inquiry." "I think his position is very difficult," Mr Miliband added.

Liberal Democrat leader Sir Ed Davey added that Ms Gray's inquiry "must interview Boris Johnson personally" to get to the bottom of claims of Downing Street parties.

Number 10 would not comment on the leaked email from Mr Reynolds at a time when the inquiry into multiple allegations of COVID rule-breaking in Downing Street and other government buildings is still ongoing.