She made the revelation about the party just before Christmas 2020 in an online post yesterday following a week of nationwide outrage focused on the Downing Street party scandals.
She said: “On the evening of December 17, 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.
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"I am truly sorry that I did this and for the anger that people will feel as a result.
"Sheffield has suffered greatly during the pandemic and I apologise unreservedly."
She said the event – which took place a day before a Number 10 Christmas party - would be considered in the context of the Cabinet Office investigation but that she did not attend any events at Downing Street.
Prime Minister Boris Johnson apologised earlier this week after admitting he too attended a gathering, at Downing Street.
On December 17, 2020, the country was subject to a tiered system of lockdown restrictions that affected people mixing and businesses being open.
London was in tier three – which meant people were banned from mixing with anyone from outside their own household or support bubble. Work Christmas lunches or parties were not permitted.
Just two days later, the city was moved to tier four restrictions where people were advised to stay at home.
A source told the Sheffield Telegraph last year that Kate Josephs had attended a party when lockdown restrictions were in place in December 2020.
But in response to our enquiries Sheffield Council said she ‘categorically did not attend a party at Downing Street.’
Journalists at The Star also made enquiries and were given a similar response. A picture Kate Josephs posted on Twitter on December 18, 2020, to mark her final day in Government shows her standing outside 10 Downing Street.
The daughter of social workers from Doncaster, Kate Josephs has worked in the White House as well as Whitehall before coming back to South Yorkshire.
She was awarded a Companion of the Order of the Bath for Public Service in the Queen’s Birthday Honours List last summer.
In her apology statement she said that she had been ‘co-operating fully with the Cabinet Office investigations and I do not want to pre-empt the findings of the investigation.’
She added: “However as chief executive of Sheffield City Council I am responsible for leading the organisation and working with partners across the city and region to support our Covid response and recovery.
"That is why I have decided to make a statement.”
National reports suggested that dozens of officials attended Kate Joseph’s leaving drinks and that it was a ‘boozy’ affair.
Sheffield Council leader, Coun Terry Fox, said: “Kate has made a statement today about a gathering that happened in the Cabinet Office last year. I have discussed this in person today with Kate on a one-to-one and she has said sorry.
"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.”