The country’s leading civil servant has stepped down from his role investigating alleged lockdown-breaking parties inside Downing Street after claims emerged he had hosted an event himself over the same period. No 10 said Simon Case had removed himself from the process “to ensure the ongoing investigation retains public confidence”. He will be replaced by veteran civil servant Sue Gray, second permanent secretary at the communities and levelling up department. “She will ascertain the facts and present her findings to the prime minister,” the statement added, giving no time for this. A report by Case into an alleged party inside No 10 on 18 December last year, when all indoor social mixing was banned, plus a series of other claimed events, had been due to be published as early as this week. But the plans were plunged into doubt after a series of sources alleged a Christmas event for staff took place in Case’s office on 17 December, the day before the supposed Downing Street party. Some outlets described staff from both the Cabinet Office and other departments mingling in Case’s office and an outside room on 17 December last year, the day before a No 10 party he has been investigating. The occasion is said to have included drinks, snacks and Christmas decorations. But a Cabinet Office statement said staff in Case’s private office took part in “a virtual quiz” on that date. It said: “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.” The disclosures are another significant embarrassment for Downing Street, which has been engulfed by a series of scandals in recent weeks, as well as the embarrassment of losing the North Shropshire byelection to the Liberal Democrats. The reports raised questions about what Case told Boris Johnson prior to taking on the inquiry – the prime minister has repeatedly insisted no rules were broken by either him or his staff. Labour had said the claims about Case made it uncertain whether he could properly look into the alleged party on 18 December, as well as another alleged event at Downing Street on 27 November, and reports of a party at the Department for Education. “Labour made it clear when the investigation was launched that the person in charge should be uncompromised and able to make a fair and independent judgment,” she said. “These fresh revelations put that into question. The SNP’s Westminster leader, Ian Blackford, said Case should be removed from the process, calling instead for “an inquiry led by an independent arbitrator – someone who owes the UK government nothing”. Asked about Case’s report earlier on Friday, Oliver Dowden, the Conservative party chairman, had said he was “confident that that inquiry will vindicate the prime minister’s assertion that everything that happened was within the rules”. The claims will add to the pressure on Downing Street over weeks of reports about parties, named by campaigners as one reason for voter disaffection at the North Shropshire byelection, where the Liberal Democrats won a previously ultra-safe Conservative seat on Thursday. They come a day after a joint investigation by the Guardian and Independent was told that Johnson joined No 10 staff during the first lockdown in May last year, raising questions about a possible longer-term culture of flouting Covid rules. The prime minister allegedly spent about 15 minutes with staff at the alleged social gathering on 15 May 2020, telling one aide inside No 10 that they deserved a drink for “beating back” coronavirus. Sources claimed about 20 staff drank wine and spirits and ate pizza following a press conference on that day, some in offices inside No 10 and others going into the garden. Some staff stayed drinking until late into the evening, they alleged.