Cabinet secretary Simon Case is no longer leading the investigation into parties which allegedly took place in Downing Street last December, after it emerged that an event was held by his own private office.
Case was investigating whether Covid rules had been broken at events organised for government staff last year, and had been expected to report his findings by the end of this week.
He stepped down from the inquiry after reports that members of his private office gathered on 17 December to take part in a quiz.
Cabinet Office confirmed that this event took place, but said the cabinet secretary merely walked through the office and “played no part in the event”.
Case has now recused himself from the investigation, according to a spokesperson for No.10, “to ensure that [it] retains public confidence”.
The remainder of the investigation will now be taken over by Sue Gray, former head of the Cabinet Office Proprietry and Ethics team.
Gray, who is currently second permanent secretary at the Department for Levelling Up, Housing and Communities, will "ascertain the facts and present her findings to the prime minister", according to the No.10 spokesperson.
Responding to the claims, a government spokesperson said: "Staff in the cabinet secretary's private office took part in a virtual quiz on 17 December 2020.
"A small number of them, who had been working in the office throughout the pandemic and on duty that day, took part from their desks, while the rest of the team were virtual.
"The cabinet secretary played no part in the event, but walked through the team's office on the way to his own office.
"No outside guests or other staff were invited or present. This lasted for an hour and drinks and snacks were bought by those attending. He also spoke briefly to staff in the office before leaving."
Institute for Government senior fellow Dr Cath Haddon has said it was the “right decision” for Case to step down.
Writing in a blog on the IfG website, she said: “The office of the cabinet secretary is one that has to be both scrupulously impartial and trustworthy and to remain would have risked undermining the authority of any future cabinet secretary inquiries. Though it still begs the question of why he did not step aside from the investigation earlier.”
She also welcomed the decision to appoint Gray to lead the remainder of the investigation saying that Gray has "conducted many of these investigations before," and is "experienced enough to tackle the difficult aspects of this case".