The head of the taskforce responsible for leading the government’s COVID-19 response held an office leaving bash at a time when indoor socialising was banned, it has been revealed.

Kate Josephs, the former director general of the Cabinet Office’s COVID taskforce, said she was “truly sorry” for the gathering on 17 December 2020.

She also revealed the event is now part of an wide-ranging investigation into multiple allegations of coronavirus rule-breaking in both Downing Street and other government buildings.

The “partygate” scandal is threatening the political future of Prime Minister Boris Johnson, who has admitted to attending a “bring your own booze” gathering himself in Number 10’s garden during the first national lockdown.

In the latest revelation of a Whitehall drinks event, Ms Josephs wrote on Twitter: “On the evening of 17 December, I gathered with colleagues that were at work that day, with drinks, in our office in the Cabinet Office, to mark my leaving the civil service.

“I am truly sorry I did this and for the anger that people will feel as a result.”

Ms Josephs left the COVID taskforce on 18 December 2020 and is currently chief executive of Sheffield City Council.

“Sheffield has suffered greatly during this pandemic, and I apologise unreservedly,” she added.

“The specific facts of this event will be considered in the context of the Cabinet Office investigation.

“I did not attend any events at 10 Downing Street.”

It has already emerged that a Christmas party was held in Whitehall on 17 December in the private office of Cabinet Secretary Simon Case.

This revelation forced Mr Case to quit as the head of the ongoing “partygate” investigation, with senior civil servant Sue Gray now leading the inquiry.

She is not expected to publish her findings until next week at the earliest.