CNN has learned that people believed to be in Johnson’s inner circle at the time mostly attended the gathering.

Downing Street has denied any breaches of rules at the gala, with a spokesman saying: “This is completely untrue. In line with the rules at the time, the Prime Minister hosted a small number of family members outside quite late.”

Johnson has been under pressure for weeks for allegedly holding summer garden parties and Christmas gatherings in Downing Street when the rest of the country was under strict coronavirus restrictions.

A report on the allegations, due to be released this week, could be the final straw for Johnson’s increasingly rebellious party.

Johnson’s approval ratings are falling, and parts of his governing Conservative Party seem increasingly to feel he is becoming a liability. Two polls in the past week showed as many as two-thirds of voters wanted him to resign.

Johnson was even forced to apologise to the Queen after news broke of a party in Downing Street the night before Prince Philip’s funeral. It was pointed out at the time that the Queen was forced to sit alone at St George’s Chapel in Windsor Castle to mourn her husband due to Covid-19 restrictions.