A top civil servant once described as “deputy God” has been tasked with rooting out the truth over “endemic” parties across Whitehall during coronavirus restrictions.

Cabinet Secretary Simon Case quit his role leading the inquiry into a number of alleged gatherings on Friday, after it emerged a quiz was held in his own department that he was aware of and spoke at.

Whitehall heavy hitter Sue Gray has been installed in his place to carry out inquiries into three alleged gatherings at Downing Street and the Department for Education in November and December last year, when indoor mixing was banned.

The terms of what was then Mr Case’s investigation said it could be widened to include any relevant allegations.

And Labour’s deputy leader Angela Rayner said Ms Gray now has the task of restoring public trust.

She said: “At the moment, people are saying ‘which department didn’t have a party?’”

She added: “It’s incredibly disappointing because we all know what was happening when these parties were going on, people couldn’t see their loved ones who were dying, and were making incredible sacrifices.

“So I do think that the investigation has to get to the bottom, but I think that the evidence already is showing that Boris Johnson has set a tone for this government and has allowed this to happen under his watch.”

She said Ms Gray should hand over any evidence of law-breaking which she uncovers to the police.

Ms Gray, who is second permanent secretary at the Department for Levelling Up, Housing and Communities, was previously director-general of propriety and ethics in the Cabinet Office from 2012 to 2018, and is seen as a figure who would not pull any punches in an inquiry.

She oversaw the Plebgate inquiry in 2012 after former chief whip Andrew Mitchell was accused of calling a policeman a “pleb” at the Downing Street gates, and was once described as “deputy God” by then Labour MP Paul Flynn in a meeting of Parliament’s Public Administration Committee the same year.

Former Tory MP and Cabinet office minister Oliver Letwin is reported to have said of Ms Gray: “It took me precisely two years before I realised who it is that runs Britain. Our great United Kingdom is actually entirely run by a lady called Sue Gray, the head of ethics or something in the Cabinet Office. Unless she agrees, things just don’t happen.”

She is also part of the panel deciding on who will be next chair of the media regulator Ofcom.

Ms Gray was once described by BBC Newsnight’s then policy editor as “the most powerful person you’ve never heard of”.

Speaking to that same programme on Friday, Tory MP Richard Holden described her as “formidable” and said she was “not a pushover”.

However, the Scottish National Party’s Westminster leader, Ian Blackford, said “having somebody else from the Civil Service marking their own isn’t good enough”.

Mr Blackford told the BBC’s Newsnight programme that “it needs to be someone from authority from outwith Government, from outwith the Civil Service. I would suggest that the best way to do that would be by having a judge-led inquiry”.

Ms Gray was brought in after the Guido Fawkes website reported on Friday that two Christmas parties were held in Mr Case’s department, the Cabinet Office, in December 2020, when restrictions were in place.

The Times reported that one of the parties was held on December 17, the day before the alleged Christmas party at Downing Street at the centre of the saga.

The newspaper reported that the event was listed in digital calendars as “Christmas party!” and was organised by a private secretary in Mr Case’s team.

On Friday evening, a Cabinet Office spokesperson said: “Staff in the Cabinet Secretary’s private office took part in a virtual quiz on December 17 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.”

After Mr Case was tasked with the investigation earlier this month, the Prime Minister’s press secretary insisted that “due diligence has been followed” when asked if the Cabinet Secretary had attended the Downing Street party at the centre of the saga.

Cabinet Office minister Michael Ellis told the Commons that Mr Case “was not at any relevant gathering”.

It comes after a string of claims about parties and gatherings held across Whitehall while London was under restrictions limiting people from meeting indoors, which Ms Rayner dubbed “endemic”.