Allegra Stratton has quit as an adviser to Boris Johnson.
In the footage obtained by ITV News, the Prime Minister's former spokeswoman and adviser Ed Oldfield, along with other aides, were filmed joking about a "fictional" Downing Street party.
During the rehearsal, filmed as part of a subsequently-abandoned plan for Ms Stratton to lead televised press briefings, one aide is heard saying: "It wasn't a party, it was cheese and wine."
"Is cheese and wine all right? It was a business meeting," Ms Stratton replied, to laughter in the room.
Ms Stratton then noted "this is recorded", adding: "This fictional party was a business meeting... and it was not socially distanced."
The Metropolitan Police confirmed officers are reviewing the leaked video in relation to "alleged breaches" of coronavirus regulations.
Speaking to broadcasters on Wednesday afternoon, she said: ”I understand the anger and frustration that people feel.
“To all of you who lost loved ones, endured intolerable loneliness and struggled with your business - I am sorry and this afternoon I have offered my resignation to the prime minister.”
She added: “My remarks seemed to make light of rules, rules that people were doing everything to obey. That was never my intention.”
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