Boris Johnson was warned about No 10 holding a drinks party in the garden during lockdown, the prime minister's former top aide Dominic Cummings says.
Mr Cummings - who has been strongly critical of Mr Johnson since he left No 10 - says the PM "waved aside" concerns about the gathering.
Mr Johnson has admitted he attended the event on 20 May 2020, but says he believed it was work-related.
No 10 said it was "untrue" to say Mr Johnson was "warned about the event".
A Downing Street spokesman added: "As he said earlier this week, he believed implicitly that this was a work event. He has apologised to the House and is committed to making a further statement once the investigation concludes."
Last week, Mr Johnson told the Commons he went to the gathering in the Downing Street garden and stayed at the drinks for 25 minutes to thank staff for their hard work.
However, Mr Cummings, who worked in No 10 at the time of the party, has insisted Mr Johnson "knew he was at a drinks party cos he was told it was a drinks party and it was actually a drinks party".
Writing in his blog, Mr Cummings added further detail about his account of the discussions leading up to the party on 20 May and said it showed "the PM lied to Parliament about parties".
The former adviser wrote that the day in 2020 was a "particularly intense shambles" of a day.
He alleged that Mr Johnson's principal private secretary (PPS), Martin Reynolds, had sent out the email inviting 100 staff to "socially distanced drinks in the No 10 garden", but "a very senior official replied by email saying the invite broke the rules".
"The PPS went to the official's office where they discussed it. The PPS declined to withdraw the invite. I told the PPS the invite broke the rules."
After discussing it, Mr Cummings claimed the PPS said he would "check with the PM if he's happy for it to go ahead", adding: "I am sure he did check with the PM."
Mr Cummings said he then challenged Mr Johnson himself. "I said to the PM something like, 'Martin's invited the building to a drinks party, this is what I'm talking about, you've got to grip this madhouse.'"
But he added: "The PM waved it aside."
"Not only me but other eyewitnesses who discussed this at the time would swear under oath this is what happened," he wrote.
Two other former Downing Street officials told the BBC they remember Mr Cummings telling them that day he had warned the prime minister not to go ahead, before the drinks gathering took place in the garden.
Deputy prime minister Dominic Raab told BBC Breakfast the PM had been clear Mr Cummings' account was "not true", adding that he previously expressed "contrition and has apologised in front of the House of Commons for some of the practices that went on at Downing Street".
However, Labour's shadow policing minister Sarah Jones said the claims were "extraordinary", and accused the government of being in chaos.
SNP Westminster leader Ian Blackford said Mr Cummings was "a key witness" and should be interviewed by Sue Gray, the senior civil servant carrying out an inquiry into gatherings on government premises during Covid restrictions.
Downing Street party row
Mr Cummings' intervention comes after Conservative MPs spent the weekend canvassing public opinion on the prime minister.
On Monday, Conservative MP Steve Baker told reporters: "My constituents at the moment are about 60 to one against the prime minister.
"I've listened very carefully to members of my [Conservative Party] association, too. There are some very strident voices in my constituency demanding that I support the prime minister.
"What I would say is I made my view very clear at the beginning of December: that there must be one rule for all."
The MP later added "it was impossible to say" if Mr Johnson would lead his party into the next general election.
The former minister is an influential voice among Conservative MPs, having previously led a powerful pro-Brexit group within the parliamentary party.
He supported Mr Johnson to become leader of the party in 2019, but has recently been critical over some of his decisions on coronavirus.
Tory MP Andrew Bridgen - who has publicly said he has no confidence in the PM - told BBC Newsnight he had received more than 1,000 emails in one day from voters, and "the overwhelming majority said that the prime minister should consider his position".
He said Ms Gray must question all witnesses including Mr Cummings and "get to the truth". "If it does transpire that she judges that the prime minister has misled parliament then clearly that's a very, very serious matter and I think it's probably career ending," he said.
Another Conservative MP, Damian Collins, said he had received hundreds of letters from constituents, adding: "A lot of people are very angry."
He told BBC Hardtalk that "by far the clear view" from party members in his constituency of Folkestone and Hythe was that "we should give the PM the benefit of the doubt until we see what's in Sue Gray's report".
But Tory MP Peter Bone said he had encountered a different reaction when canvassing constituents. "They were wholly supportive of the prime minister," he said. "Many of them were saying, 'well hang on a minute, look he's delivered Brexit, he's got us the vaccination, he's got us through Covid... Why on earth would we want to change the prime minister?'"
For a Conservative Party leadership contest to be triggered, 54 Conservative MPs must write to the chairman of the 1922 committee - a group made up of all backbench Tory MPs - to say they no longer have confidence in the prime minister.
On Sunday, former minister Tim Loughton became the sixth Tory MP to call on Mr Johnson to resign, saying his position was "untenable".
It is reported that those around Mr Johnson have started "Operation Save Big Dog", which could include an overhaul of his top team, following criticisms of the culture within Downing Street.
But Education Secretary Nadhim Zahawi told BBC Breakfast: "Honestly, I don't recognise that at all."
Mr Johnson's official spokesman also dismissed reports of "Operation Red Meat" - rushing out policies popular within the party to bolster the PM - saying: "None of these issues are things that we have not been seeking to address for some time."
Labour leader Sir Keir Starmer told LBC radio Mr Johnson was "too weak to lead... he has lost all authority and that matters, whatever party you are in," he added.
Sir Keir has said a picture of him drinking beer in an office last spring did not show a breach of Covid rules in place at the time.
The Labour leader said the photograph, which first emerged last year, was of him in a constituency office in the run-up to the Hartlepool by-election.
"There is simply no comparison" to the culture within Downing Street, Sir Keir said, adding that Conservatives bringing it up were trying "to take everyone into the gutter with them".
But Mr Zahawi said he hoped Sir Keir "finds [it] within himself to apologise" over the image.
Timeline: The alleged government gatherings
The government is facing mounting pressure over several events that are alleged to have been held during lockdowns. Here is what we know about them and the restrictions in place at the time:
10 May 2020
Boris Johnson announced a plan to take the “first careful steps" out of the lockdown that began in March 2020. But he said people should continue to "obey the rules on social distancing and to enforce those rules we will increase the fines for the small minority who break them”.
Legal restrictions at the time said you could not leave your house without a reasonable excuse and government guidance was that you could meet one person outside of your household in an outdoor setting while exercising.
15 May 2020
A photo from May 2020 showed the prime minister and his staff with bottles of wine and a cheeseboard in the Downing Street garden. When asked about it, Boris Johnson said, “those people were at work talking about work”.
20 May 2020
About 100 people were invited by email to “socially distanced drinks in the No 10 garden” on behalf of the prime minister’s principal private secretary, Martin Reynolds.
Witnesses told the BBC the PM and his wife were among about 30 people who attended.
Boris Johnson has confirmed he attended the event, saying he was there for 25 minutes and “believed implicitly that this was a work event”.
17 July 2020
Boris Johnson announced plans for a “significant return to normality" in England by Christmas "through targeted, local action” instead of national lockdowns.
But he added that the timetable relied on “every one of us staying alert and acting responsibly”.
5 November 2020
With cases of coronavirus rising again, the prime minister told people in England that “we are once again asking you to stay at home” as a new national lockdown began.
He said people should only leave their homes “for work if you can’t work from home, for education, and for essential activities and emergencies”. Indoor gatherings with other households were banned, unless they were for work purposes.
13 November 2020
Sources told the BBC that Downing Street staff members attended a gathering with Carrie Johnson in the flat where she and the prime minister live. A spokesman for Mrs Johnson denies the party took place.
27 November 2020
A leaving event was held for No 10 aide, Cleo Watson, where people were drinking, and Mr Johnson made a speech, according to sources.
2 December 2020
The second national lockdown ended after four weeks but Boris Johnson replaced those restrictions with “tough tiers to keep this virus down”.
London was placed in tier two, which banned two or more people from different households from meeting indoors, unless “reasonably necessary” for work purposes.
10 December 2020
The Department for Education has confirmed it had an office gathering to thank staff for their work during the pandemic. It says drinks and snacks were brought by those who attended and no outside guests or support staff were invited.
14 December 2020
The Conservative Party has admitted that an “unauthorised gathering” took place at its HQ in Westminster. It was held by the team of the party's London-mayoral candidate, Shaun Bailey, who has since stepped down as chair of the London Assembly police and crime committee. The Metropolitan Police is to speak to two people who attended the party.
15 December 2020
Multiple sources have told the BBC there was a Christmas quiz for No 10 staff last year. A photo - published by the Sunday Mirror - showed Boris Johnson taking part and sitting between two colleagues in No 10. Mr Johnson has denied any wrongdoing.
16 December 2020
London moved into the highest tier of restrictions and Matt Hancock, who was health secretary at the time, said it was important “everyone is cautious” ahead of the festive period.
The Department for Transport apologised after confirming reports of a party in its offices that day, calling it “inappropriate" and an "error of judgment” by staff.
17 December 2020
A leaving party was held at the Cabinet Office for the outgoing head of the civil service Covid taskforce - the team responsible for drawing up coronavirus restrictions.
Kate Josephs, now chief executive of Sheffield City Council, apologised for the event, saying she was “truly sorry that I did this and for the anger that people will feel as a result”.
18 December 2020
Downing Street originally denied a report by the Daily Mirror that a party took place in Downing Street.
However, a video obtained by ITV News showed the prime minister's then-press secretary Allegra Stratton, joking about reports of an event, saying: “This fictional party was a business meeting and it was not socially distanced.”
12 April 2021
Lockdown restrictions were eased in England, with pubs and restaurants allowed to reopen with outdoor service only.
However, working from home continued to be recommended and socialising indoors with people from other households was not allowed. Meeting others outdoors was limited to groups of six people or two households.
16 April 2021
Two parties were held by Downing Street staff at No 10, the night before Prince Philip's funeral.
One of the events was a leaving party for the PM's then director of communications James Slack, who has apologised for the event and acknowledged it “should not have happened at the time that it did”.
Boris Johnson was not at either party.