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Following political calls for officers to investigate allegations relating to May 2020, the Metropolitan Police said it was liaising with the Whitehall department over the latest claims.
Labour has accused Mr Johnson of ignoring “rules he puts in place for the rest of us” after a leaked email appeared to show Martin Reynolds arranging the drinks gathering in the garden at No 10 Downing Street during lockdown.
Mr Reynolds, Mr Johnson’s principal private secretary, sent an email to more than 100 Downing Street employees asking them to “bring your own booze” for an evening gathering, ITV reported.
He said they should “make the most of the lovely weather”, despite England being under tough Covid-19 restrictions in May 2020.
At the time, the government was ordering ordinary members of the public not to meet, even outdoors, and tight restrictions were in place on social mixing, including at funerals.
Mr Johnson imposed England’s first lockdown against Covid-19 in March 2020 and it was not until June 1 that groups of up to six people were allowed to meet outdoors.
Deputy Labour leader Angela Rayner said Mr Johnson “should be ashamed” after more claims of rule-breaking at No 10, which she described as “despicable”.
The email from Mr Reynolds relates to an event said to have taken place on May 20, 2020.
Allegations of that gathering, said to have been attended by 40 people, emerged last week when Dominic Cummings, a former senior aide to Mr Johnson, said he had issued a warning at the time that the “socially distanced drinks” would probably be against the rules and “should not happen”.
The Sunday Times and ITV have claimed the prime minister attended the event with his wife, Carrie Johnson, whom he was engaged to at the time.
What the UK looked like on May 20, 2020 - in pictures
Human rights lawyer Adam Wagner, who interprets coronavirus regulations on Twitter for the public, said the event looked “unlikely to be legal” for those who attended.
“Being outside the home was illegal at the time unless it was for the need to work,” Mr Wagner posted on social media.
He said that “socially distanced drinks” and “bring your own booze” did not sound like work.
Ms Rayner said Sue Gray, the senior civil servant investigating claims surrounding lockdown-breaking gatherings in government, should confirm that the leaked email would form part of her inquiry.
The Cabinet Office official has already confirmed Ms Gray is investigating the May 20 event, along with an alleged garden gathering five days earlier.
The earlier gathering was revealed by a leaked photo showing Mr Johnson and staff sitting around a table with cheese and wine.
“It is 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 No 10 were breaking the rules,” Ms Rayner told ITV News.
“It is disgraceful and he should be ashamed.
“If he has clearly broken the law, then that should be referred to the police and the police should handle it. Nobody is above the law.
“Many people at the time could not go to funerals, were not able to meet families and friends, and they will be disgusted by this because not only does it seem Boris Johnson is breaking his rules, but he has been lying to people as well, and this is completely unacceptable and not what you would expect from the British prime minister.”
Ms Rayner said Mr Johnson had “consistently shown that he has no regard for the rules he puts in place for the rest of us”.
During a raucous parliamentary debate on Tuesday, Labour put forward an Urgent Question on the alleged May 20 gathering.
Mr Johnson whereabouts were scrutinised after he opted against responding to the question, with Cabinet Office minister Michael Ellis being sent out to respond in his place.
Mr Ellis was relentlessly heckled by opposition MPs as he confirmed the May 20 “bring your own booze” event was being investigated. But he noted that the publication of the inquiry’s findings could be delated owing to the increasing number of allegations to be looked at.
Asked by Labour MP Abena Oppong-Asare if the prime minister would resign if he was found to have knowingly misled Parliament about the existence of the May 20 gathering or any other parties, Mr Ellis said “there’s absolutely no indication” that Mr Johnson knowingly misled lawmakers.
Ms Rayner said it “won’t wash to blame this on a few junior civil servants” because the prime minister “sets the tone” for the conduct of his administration.
Speaking before the parliamentary debate, Liberal Democrat leader Sir Ed Davey said: “This is yet more evidence that while the vast majority of people were sticking to the rules, those in No 10 were breaking them.
“To add insult to injury, on the very same day that the culture secretary said people could only meet in pairs outdoors, it seems Boris Johnson’s staff were holding a boozy party in Downing Street.”
Mr Davey said Ms Gray’s inquiry “must interview Boris Johnson personally” to get to the bottom of claims of Downing Street parties.
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.
“My dad died just four days before this email was sent out,” Ms Brady said. “He was only 55 and was a fit and healthy key worker.
“Those days will stay with me for the rest of my life. Just like the families of the 353 people that died that day, my family could not even get a hug from our friends.
“To think that while it was happening Boris Johnson was making the ‘most of the weather’ and throwing a party for 100 people is truly beyond belief.”
Amid mounting pressure on Mr Johnson on Tuesday, Labour leader Sir Keir Starmer urged the prime minister to “finally come clean” on the string of alleged gatherings that are said to have breached coronavirus regulations.
He accused him of resorting to “absurd” measures to distract from the scandal and said he had been dishonest with Britons.
“Boris Johnson, your deflections and distractions are absurd,” Sir Keir tweeted. “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.”
A sleaze watchdog accused Mr Johnson’s administration of being “careless” over its approach to maintaining proper standards.
Lord Evans of Weardale, chairman of the Committee on Standards in Public Life, highlighted the row centred on “partygate” as one of the causes of concern.
He said the public expects Members of Parliament and officials to be “living up to the standards that they profess to live up to”.
Health Minister Ed Argar said “appropriate disciplinary action” should be taken if rules were found to have been breached at the alleged drinks party.
“I can entirely understand why people who have lost loved ones or people who have just had their lives hugely disrupted by these restrictions are angry and upset by these allegations,” he told BBC Breakfast.
“That is why it is right that [Sue Gray] is looking into the facts and will report, and she can go with this investigation where she needs to, without fear or favour.
“I hope that she will report swiftly and, depending on what she finds in that investigation — if people are found to have broken the rules in that context, it is right that appropriate disciplinary action is taken.”
Asked by Sky News if the prime minister should resign if the probe finds he attended the event in question, Mr Argar declined to say what he thought the appropriate response would be.
“I think it is important we wait and see what [Ms Gray] says about the facts,” he said.
Pressed on whether he would have “stepped in” if he had seen 40 people gathering in the garden at Downing Street, Mr Argar refused to be drawn on the “hypothetical” question.
The latest development in the parties saga engulfing the Conservative-led government will pile further pressure on Mr Johnson and weaken his authority.
Late last year, after reports first surfaced about the alleged breaches of Covid restrictions, the prime minister was told by MPs that his reputation as a leader was on the line and the public’s trust had waned.
The crisis caused poll numbers to tumble for the Conservatives as many traditional Tory voters voiced their anger over the party allegations.
As a wave of Omicron swept across the country, Mr Johnson was warned the public would not listen to him or abide by new lockdown rules if they were introduced.
After introducing his “Plan B” measures — which include mandatory face masks in most public places, working from home and vaccine certificates in nightclubs — Mr Johnson said the restrictions were enough to protect the NHS. He stopped short of imposing another nationwide shutdown.
No 10 said it would not comment on the party claims while Ms Gray’s investigation was under way.