THE man tasked with investigating whether Downing Street Christmas parties broke Covid lockdown rules "hosted Christmas parties" in his Whitehall office, it has been alleged.

Cabinet Secretary Simon Case - the highest ranking civil servant in the UK - had two separate parties in his office, adjacent to Downing Street, reports have claimed.

The right-wing Westminster gossip website Guido Fawkes broke the story, claiming that “parties were held in Case’s own 70 Whitehall office on two occasions last December”.

The site claimed that “rumours” of the events had been circulating, but a whistleblower had confirmed that at least one had involved around 15 people and “copious booze and music”.

While Case reportedly did not attend, he allegedly stuck his head in several times.

READ MORE: Boris Johnson 'went to Downing Street party while UK was in Covid lockdown'

The second event allegedly “saw staff drinking in Case’s office before departing for drinks elsewhere”.

According to reports from the BBC and The Times, the digital calendar notification sent out for the event on December 17 was titled "Christmas party!".

Last week, Boris Johnson asked Case to look into whether government staff had broken lockdown rules with Christmas parties held in 2020. At the time, Christmas parties and gatherings were expressly prohibited.

The Prime Minister suggested that the remit of Case’s investigation could be extended to cover other events if the need arose.

The allegations have led Ian Blackford, the SNP’s leader at Westminster, to write to the Prime Minister claiming the investigation had been “fundamentally and fatally undermined”.

The National: EDITORIAL USE ONLY. NO SALES. NO ALTERING OR MANIPULATING. ..MANDATORY CREDIT: UK Parliament/Jessica Taylor..UK Parliament handout photo of SNP Westminster leader Ian Blackford during Prime Minister's Questions in the House of Commons, London.

Blackford (above) wrote: “The Cabinet Secretary’s internal investigation cannot be trusted and it must now be scrapped. The public cannot possibly have any confidence in an inquiry, led by someone alleged to have been involved in the very matter being investigated.

“Independence of review and absolute transparency is of paramount importance in this matter.

“It is clear that the Prime Minister must now ensure a judge is able to lead an inquiry that is independent and external of this Government. This judge-led inquiry must act at pace and review all allegations of impropriety and rule breaking that have taken place across Government.

“Given that the Met Police were awaiting the Cabinet Secretary to pass on any further evidence emerging as a result of your internal inquiry to determine any potential criminal wrongdoing, it is also clear that the police now must look again at the overwhelming body of evidence.”

The Met Police have previously been threatened with legal action if they fail to adequately explain their decision not to investigate the slew of alleged events held across Whitehall while the rest of the UK sat in lockdown.

The latest news is likely to bring further accusations of the Tory government “marking its own homework”.

READ MORE: Cabinet Secretary defends PM ‘marking own homework’ on flat renovation probe

Such accusations previously followed news that Boris Johnson, as the ultimate arbiter of the ministerial code, would decide whether he was guilty of any wrongdoing over the funding of flat renovations at 11 Downing Street.

Speaking in May, Case defended Johnson’s role as the “judge and jury” of whether he had committed an offence, saying it was “one of our basic constitutional principles”.

In a ruling earlier this December, the Electoral Commission said the Tory Party had not followed the law when funding those flat renovations.

Furthermore, the information uncovered by the Commission contradicted what the Prime Minister had told his own standards tsar - leading to accusations of Johnson having deliberately “lied”.

In a recent update, Downing Street said that Case had "recused himself" from the investigation, to be replaced at its head by Sue Gray, the former Cabinet Office director-general of the propriety and ethics team.

In a statement, the Cabinet Office accepted Case had known about one event held in his private office, but did not address allegations of a second.

The 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.”