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At the time, outdoors gatherings were banned in England under the strict Covid curbs.
Prime Minister’s Principal Private Secretary Martin Reynolds sent the invitation email which has since been leaked to the news organisation.
Among those invited include the Prime Minister’s advisors, speechwriters and door staff.
As obtained by ITV News, the email read: “Hi all, After what has been an incredibly busy period it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No10 garden this evening.
“Please join us from 6pm and bring your own booze!”
Several staff voiced concerns over the invite, reports the BBC, with a source telling the broadcaster: “I remember people saying, 'What the hell?'"
Late Monday night, a Scotland Yard spokesperson said the Met Police was in contact with the Cabinet Office over the reports.
A spokesperson 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."
No 10 said it would not comment on the allegations while Sue Gray, a senior civil servant, carries out her inquiry into numerous allegations of rule-breaking events being held in Downing Street during the coronavirus pandemic.
Labour’s deputy leader, Angela Rayner, said the email should form part of Ms Gray’s inquiry.
She told ITV News: “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 actually told to follow the rules, Boris Johnson and No 10 were breaking the rules.
"It is disgraceful and he should be ashamed."
It follows allegations of a separate staff gathering in the garden on May 15.
A photograph emerged of Mr Johnson and his wife sat with No 10 staff drinking wine and eating cheese.