Mr Johnson attended the event and gave a speech. The Mirror, which first reported the event before the police investigation began, said Mr Johnson was only there “for a few minutes”.

Tier three restrictions had been implemented the day before in London, meaning people were not allowed to mix indoors with anyone outside their household or support bubble.

Capt Higham is said to have had a close personal relationship with Mr Johnson, who took part in interviews to select the military adviser’s replacement. Capt Higham became Commanding Officer of the HMS Prince of Wales in July 2021.

The third newly-emerged event took place on January 14 2021, described by Ms Gray as “a gathering in No 10 Downing Street on the departure of two No 10 private secretaries”.

The Telegraph understands a senior civil servant in the culture department, who took up the role after leaving Downing Street, was one of the two officials. The other’s identity is not known.

Mr Johnson is also understood to have attended the leaving event. It means the publicly known list of gatherings the Prime Minister was at which are being looked into for Covid lockdown breaches has grown again.

At the time, the UK had just been plunged back into a third lockdown, with people prohibited from holding parties or mixing indoors with people outside of their household.

Ms Young and Capt Higham were approached for comment via spokesmen at the government bodies where they work. No comments were issued.

Downing Street, the Cabinet Office, the Metropolitan Police and government bodies where the three individuals work all declined to comment.

A Foreign Office spokesman said: “It would not be appropriate to comment while a Met Police investigation is ongoing.”