In the letter to Dame Cressida, Sir Ed wrote: “It is now vital that you take urgent action to investigate potentially unlawful behaviour on the part of all those who attended the party, including Boris Johnson.

“On the day of the Downing Street garden party, the Metropolitan Police Twitter account published a tweet which stressed the importance of continuing to follow the lockdown rules in place at the time, which included not meeting with more than one other person outdoors.

“It is clear from the descriptions of the Downing Street party that it failed to meet these criteria. 

“It also was clearly not a work meeting – given the instruction to ‘bring your own booze’. It was therefore almost certainly an illegal gathering.”

‘Treat everyone equally under law’

Sir Ed pointed out that more than 17,700 people had been fined by the Metropolitan Police for breaching Covid laws during the pandemic. He said it was vital everyone was treated equally under the law.

“People have sacrificed so much during this pandemic, from cancelling weddings to not being able to say goodbye to their loved ones in hospital,” he said.

“The police must reassure the public that justice will be done and there isn’t one rule for them and another for Boris Johnson and his colleagues in Downing Street.”

Allegations regarding the event on May 20 2020 emerged on Monday afternoon, when an email from Martin Reynolds, the Prime Minister’s principal private secretary, was published.

In it, Mr Reynolds, a senior civil servant, who has run Mr Johnson’s private office since October 2019, wrote: “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.”