No fines were issued when it was initially investigated, but a video recently broadcast by British media, which showed people dancing and partying at the event, has prompted officers to look again.

Police said their approach was only to retrospectively launch investigations when there was evidence of a serious and flagrant breach and other conditions were met.

The Conservative Party did not issue a fresh comment following the police announcement, and referred to a previous statement in which it said the gathering was unauthorised and that four staff had been disciplined as a result.

An additional investigation was opened into a December 8, 2020 gathering in parliament, but police did not provide details about those in attendance.

Police also said they had assessed information provided by the Cabinet Office regarding potential breaches of the regulations between June 2020 and May 2021 at Johnson’s then-Downing Street residence and office, and the prime ministerial country home, Chequers.

They had “concluded that they do not meet the retrospective criteria for opening an investigation”, police said.