The UK’s top civil servant is expected to step aside from his role leading an inquiry into Downing Street lockdown parties, after it emerged an event was held in his own office.
Simon Case had been due to report on claims Covid rules were broken at events for staff last year.
But his role was put into doubt after reports a party was held in his office, while London was under Covid rules.
No 10 is expected to issue a statement later on Mr Case’s position.
The BBC understands a quiz was held for members of Mr Case’s private office on 17 December 2020, and invites were sent out titled “Christmas Party!”
About 15 people were invited to the gathering at 70 Whitehall, near Downing Street, although it is understood not everyone turned up.
‘Corruption and sleaze’
The BBC has been told that while Mr Case did not take part in the Christmas quiz, he was present for the event.
Staff who were in the office that day took part from their desks, and others joined remotely.
There was no catering, but people consumed drinks and snacks which they bought.
The party was first reported on the Guido Fawkes website on Friday afternoon.
Mr Case, who has been cabinet secretary since last September, has been asked by Boris Johnson to investigate gatherings for staff in government buildings last year.
His inquiry is focusing on events in Downing Street on 27 November, 15 December and 18 December, and at the education department on 10 December.
Responding to press reports of the gathering in Mr Case’s office, Labour’s deputy leader Angela Rayner said: “Boris Johnson as prime minister has set the tone for the civil service and the rest of government.
“With each new revelation there is growing evidence of a culture of turning a blind eye to the rules.
“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 judgement. These fresh revelations put that into question.”
The SNP’s Westminster leader Ian Blackford said Mr Case’s investigation “cannot be trusted and it must now be scrapped”.
He urged the prime minister to appoint a judge, independent of government, to lead the inquiry.
Speaking to the BBC he said the Mr Case had been allowed to “mark his own homework” and that his position was “no longer tenable”.
“This is a government that stinks of corruption and stinks of sleaze,” he added.