Simon Case, the UK’s most senior civil servant, will oversee any action arising from the probe into lockdown-busting parties, despite his withdrawal as head of the investigation following claims of parties in his own office.

       Boris Johnson is under intense pressure after a leaked email showed that No 10 staff were invited to a drinks party in May 2020, when people were banned from meeting more than one person outdoors.

       Mr Johnson has refused to say whether he was present at the gathering, while Sue Gray, a senior civil servant, carries out an investigation into a series of allegations about rule-breaking parties at Downing Street and other government departments.

       Ms Gray’s inquiry is expected to focus on establishing the facts, leaving it to the prime minister to determine any consequences for his own position and political staff, while Mr Case is in charge of the fallout for the civil service, The Independent and Politico understand.

       As cabinet secretary and head of the civil service Mr Case is the ultimate arbiter of any disciplinary action faced by civil servants or proposals for culture change in Whitehall. He is also the line manager for Martin Reynolds, the senior aide who emailed staff about a “bring your own booze” gathering in the garden of No 10.

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       Mr Case was originally placed in charge of the investigation into Downing Street parties, but recused himself in December after reports of social gatherings in his own department.

       Civil service officials claimed he had attended drinks outside his personal office in 70 Whitehall in December 2020, as first reported by The Independent and Politico — allegations which were categorically denied by the Cabinet Office. Mr Case was later accused of misleading staff about his knowledge of the event.

       Separately, The Times reported that Mr Case attended another Christmas party in his department after London had been placed in Tier 3 restrictions. The Cabinet Office acknowledged a virtual quiz had taken place but said Mr Case “played no part in the event”.

       Announcing Mr Case’s decision to step aside from the inquiry, a No 10 spokesperson said he had done so “to ensure the ongoing investigation retains public confidence”.

       It is not known whether Mr Case attended or had knowledge of any other parties which may have contravened COVID-19 restrictions, including those at Downing Street.

       However, Whitehall staff have since raised concerns over his role in enforcing any disciplinary measures or institutional reforms after Ms Gray has published her report.

       One official who attended the 70 Whitehall party pointed out that Ms Gray was technically answerable to Mr Case and it was “ridiculous” he would oversee any ensuing steps.

       Another said that the civil service ought to be able to avoid “these conflicts of interest.” They added that there needed to be a “clear route” to avoiding anyone “working directly with the prime minister and his office” or who had knowledge or took part in any parties from making decisions about the investigation.

       At present, the “cabinet secretary would effectively be either marking his own homework or marking the prime minister’s,” they said.

       Catherine Haddon, senior fellow at the Institute for Government, said Ms Gray’s report would most likely focus on the facts rather than any sanctions for individuals, “but the kind of language she uses is going to imply something about the leadership not only of the prime minister, but also by leaders in the civil service”.

       She added: “Case himself is going to have questions to answer, because these are civil servants as well. And if he knew it was happening, then ultimately the responsibility lies with him.”

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       Thus far, Ms Gray has focused her time and interviews on senior staff and those already named in press reports, according to several officials with knowledge of the process. She has insisted on interviewing the prime minister as part of her enquiries, the same officials said.

       A Cabinet Office spokesperson said: “We are not going to speculate on the outcome of an ongoing investigation.”