The Prime Minister's administration was forced to apologise on Friday to Buckingham Palace after it emerged two staff parties were reportedly held on the eve of the Duke of Edinburgh's funeral.
Sue Gray, the senior civil servant investigating the litany of claims about lockdown-busting parties in Government, is said to have been "completely blindsided" by the latest revelations, The Times reported.
The newspaper said the official is concerned that Downing Street staff are withholding information about parties from her after it was claimed on Friday staff gathered after work for two events on April 16 2021, which started separately and later merged.
Witnesses said alcohol was drunk and guests danced to music, with a person sent to a local shop with a suitcase to buy wine, according to The Telegraph, which first reported the allegations.
Meanwhile, further claims emerged on Friday evening of regular "wine time Friday" gatherings in No 10 while coronavirus rules were in place.
The Mirror said the weekly events are a "long-standing Downing Street tradition", including under previous administrations, but that they continued after Covid restrictions were introduced prohibiting indoor mixing between households.
The newspaper said staff invested in a £142 drinks fridge to keep their bottles of alcohol chilled, and that the Prime Minister was aware of the socialising.
A No 10 spokeswoman said: "There is an ongoing investigation to establish the facts around the nature of gatherings, including attendance, setting and the purpose with reference to adherence to the guidance at the time.
"The findings will be made public in due course."
It comes as reports suggested Mr Johnson was mounting a fight back after a tumultuous week, with The Times and Daily Telegraph both stating that he has been in contact with Lynton Crosby, the political strategist who helped get elected as mayor of London.
Mr Johnson will look to move on from so-called partygate by lifting Plan B measures, such as the guidance to work from home, at the end of the month, the Telegraph reported.
The restrictions, which were designed to slow the spread of the Omicron variant of coronavirus, are due to expire on January 26, subject to a review.
Levelling-Up Secretary Michael Gove hinted that there could be sackings as part of a shake-up at the top of Government, but rejected a suggestion that Mr Johnson should be one of those to go.
The Cabinet minister, referencing the need for Ms Gray's inquiry to establish what went on, told ITV News: "If there is a specific need for disciplinary action or for responsibility to be taken, let's do that, let's do it quickly, but let's also do it with all the facts in front of us."
Mr Johnson has continued to face criticism, including from those on his front bench, after admitting to attending a "bring your own booze" garden party in No 10 on May 20 2020, which he insists he understood to be a "work event".
Pensions minister Guy Opperman said the Prime Minister "needs to change his ways" as he recalled how, in the same month when some in No 10 were "not behaving appropriately", he was not able to support his children before they died.
He and his wife Flora's newborn twin boys, Teddy and Rafe, died the next month.
Speaking to BBC's Politics North programme, the Tory MP said: "I feel pretty emotional about this because in May 2020 my wife and kids were unwell and they went to hospital.
"I was not able to go there to support them."
Meanwhile, Labour leader Sir Keir Starmer is set to use a speech on Saturday to heap more pressure on the under-fire Prime Minister.
He is expected to tell the Fabian Society conference: "We are witnessing the broken spectacle of a Prime Minister mired in deceit and deception, unable to lead."
Sir Keir will make his speech with his party enjoying a surge in the polls, with one survey recording that Labour have risen to their largest predicted vote share in almost a decade.
A Savanta ComRes survey, its first voting intention poll since the partygate allegations broke, put Labour on 42 points, up by five, with Boris Johnson' s Tories on 32, down by one.
The Daily Mail, published an image of Sir Keir - which first emerged in spring last year - drinking a bottle of beer while talking to party staff in an office while on the local election trail last year when indoor mixing was not permitted, unless it was for work.
Labour, who have been approached for comment, have previously said the picture was from a workplace event, with Sir Keir and "a local MP" having "paused for dinner as the meeting was during the evening".