Sheffield Council boss Kate Josephs said she was "truly sorry" for her involvement in the event, which took place on December 17, 2020 - the same day as the reported Cabinet Office 'Christmas quiz' was held which involved Cabinet Secretary Simon Case. The two events were separate from each other.
Halifax-born Mrs Josephs took over as Sheffield City Council chief executive earlier this year after the job was advertised with a salary of £190,000 a year.
Her statement was posted on Twitter minutes before the Daily Telegraph published details of the event, which is reported to have been attended by "dozens" of officials from the Covid-19 taskforce which Mrs Josephs had led as director-general and involved the consumption of alcohol.The events of the leaving do will form part of civil servant Sue Gray's expanding investigation into a series of lockdown-breaking parties in Government.
Mrs Josephs wrote: "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 people will feel as a result.
"Sheffield has suffered greatly during this pandemic and I apologise unreservedly.
"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.
"I am grateful for the ongoing support of colleagues and partners and need now to ask that people allow the Cabinet Office to complete its investigation."
The event came 24 hours before the Downing Street Christmas party, which sparked the resignation of Boris Johnson's spokesperson Allegra Stratton late last year after video emerged of her joking about the event.
At the time of the event London had just entered Tier Three restrictions, with people banned from mixing indoors with anyone outside their household or support bubble.
On the day of the party, the Government tweeted official guidance stating: “Although there are exemptions for work purposes, you must not have a work Christmas lunch or party, where that is a primarily social activity and is not otherwise permitted by the rules in your tier."
When she was interviewed by The Yorkshire Post in February, Mrs Josephs said she had led "a pretty small team of incredibly committed civil servants" on the taskforce. She described her role as pulling together the work being done on Covid-19 policies by many different government departments and taking the lead on set-piece moments like the presentation by the Prime Minister of 'roadmaps' describing a path out of lockdown.
She said of her time on the taskforce: "History and hindsight will judge all sorts of things about this last period but what I've taken from it is a lot of people across the whole system working really, really hard to do the best they can. I can't think of a better way to have finished my 20 year career."
Sheffield City Council leader Terry Fox said the chief executive has discussed with him what happened and apologised face-to-face.
He said: “To be honest I need to express my deep disappointment – people will rightly feel angry and let down. I get that completely.
“After everything that Sheffield has been through during the pandemic, this is news that we did not want to receive.
“Over the last year, Kate’s been an asset to our city and she is working tirelessly for Sheffield and it’s people.
“Our residents will understandably have questions and concerns, and we await the findings of the investigation.”
Former Sheffield Council leader Paul Scriven, who is now a peer, called for Mrs Josephs to resign.
“She has no moral authority to be the senior officer of the organisation that leads on public health and Covid in my city,” he said.
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