It comes after it emerged on Friday that a garden party had been hosted in No 10 on May 20, 2020, five days after Mr Johnson, his wife and a series of senior aides were photographed drinking in the same area at a separate gathering.

Three sources told The Telegraph that the invitation for the event was sent by Martin Reynolds, the Prime Minister’s Principal Private Secretary, with attendees also encouraged to “bring your own booze”.

Dominic Cummings, the former No 10 senior adviser, claimed he thought at the time that the gathering broke lockdown rules and should not have gone ahead.

‘There were 40 people in the garden, including Boris and Carrie’

The Sunday Times also quoted a witness who claimed that 40 people had attended the event, including Mr and Mrs Johnson.

“There were 40 people in the garden, including Boris and Carrie. There were long tables laden with drink, crisps, sausage rolls and other picnic food. It was a proper booze-up,” they said. 

With Ms Gray’s probe expanding over the weekend to take account of both the May 15 and 20 events, three sources said they believed it was now inevitable that Mr Johnson would be dragged into her investigation.

However, one said it was possible that she had already questioned Mr Johnson about the events, noting that some members of staff had already been questioned about the use of the Downing Street garden early last week.