The revelations came in a leaked email seen by British broadcaster ITV.

At the time, these were the rules in Britain, as explained by the PM himself just 10 days before the alleged gathering:

"You can sit in the sun in your local park, you can drive to other destinations, you can even play sports but only with members of your own household. You must obey the rules on social distancing and to enforce those rules we will increase the fines for the small minority who break them."

Police prosecuted people for having parties, erected random checkpoints in some areas and in Derbyshire, central England and used drones to monitor beauty spots.

ITV reported that around 40 members of staff attended the event on May 20, 2020, including the Prime Minister and his partner Carrie.

This is the latest of at least 5 allegations that government departments broke lockdown restrictions - and are now under investigation by senior government official Sue Gray.

Asked about the claims of parties at No.10, Johnson told parliament last month that all COVID guidance had been followed, no rules had been broken and that there had been no party in Downing Street.

British minister Michael Ellis answered an urgent question to the prime minister in parliament about the party on Tuesday (January 11).

"I apologize again unreservedly for the upset that these allegations have caused. The prime minister has asked for an investigation to take place and the terms of reference for the investigations that are underway have already been published.''

In response to a question asking whether Johnson would resign if he is found to have broken the law, he added:

"The prime minister is going nowhere. The prime minister retains the confidence of the people of this country".