Boris Johnson has "categorically" denied he was warned a drinks party in the No 10 garden risked breaking lockdown rules.
"Nobody warned me that it was against the rules," the prime minister said, adding: "I would have remembered that."
Former aide Dominic Cummings says he warned Mr Johnson at the time, and has accused him of misleading MPs about it.
Asked if he would resign if he was found to have misled MPs, Mr Johnson said: "Let's see what the report says."
Senior civil servant Sue Gray is due to publish a report into alleged Covid rule breaking at a lockdown get-togethers in Downing Street and government departments.
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.
But Deputy Prime Minister Dominic Raab told BBC Breakfast the PM had been clear Mr Cummings' account was "not true", adding that he had acted in "good faith" and previously expressed "contrition and has apologised in front of the House of Commons for some of the practices that went on at Downing Street".
Mr Raab conceded that the accounts from Mr Cummings and the PM "cannot be reconciled" but refused to speculate further while Sue Gray's investigation was still ongoing.
When questioned on Mr Johnson's stability as leader, Mr Raab said he was confident the PM would "carry on for many years and into the next election".
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 Ms Gray, the senior civil servant investigating gatherings on government premises during Covid restrictions.
The authorities are going through mounting strain over a number of occasions which are alleged to have been held throughout lockdowns.
Here is what we know about them and the restrictions in place at the time:
10 May 2020
Boris Johnson introduced 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 stated you could possibly not depart your own home without good reason and authorities guidance was that you could possibly meet one specific person outside of your family in an out of doors setting while exercising.
15 May 2020
A photograph from May 2020 confirmed the prime minister and his workers with bottles of wine and a cheeseboard within the Downing Street backyard. When requested about it; Boris Johnson stated “those people were at work talking about work”.
20 May 2020
About 100 folks had been invited by e mail to “socially distanced drinks in the No 10 garden” on behalf of the prime minister’s principal non-public secretary; Martin Reynolds.
Witnesses informed the BBC the PM and his spouse had been amongst about 30 individuals who attended.
Boris Johnson has confirmed he attended the occasion; saying he was there for 25 minutes and “believed implicitly that this was a work event”.
17 July 2020
Boris Johnson introduced plans for a “significant return to normality” in England by Christmas “through targeted; local action” as an alternative of nationwide lockdowns.
But he added that the timetable relied on “every one of us staying alert and acting responsibly”.
5 November 2020
With case of coronavirus rising once more; the prime minister informed folks in England that “we are once again asking you to stay at home” as a brand new nationwide lockdown started.
He stated folks ought to solely depart their properties “for work if you can’t work from home; for education; and for essential activities and emergencies”. Indoor gatherings with different households had been banned; except they had been for work functions.
13 November 2020
Sources informed the BBC that Downing Street workers members attended a gathering with Carrie Johnson within the flat the place she and the prime minister dwell.
A spokesman for Mrs Johnson denies the occasion occurred.
27 November 2020
A leaving occasion was held for No 10 aide; Cleo Watson; and Mr Johnson made a speech; in response to sources.
2 December 2020
The second nationwide lockdown ended after 4 weeks however Boris Johnson changed these restrictions with “tough tiers to keep this virus down”.
London was positioned in tier two; which banned two or more folks from totally different households from assembly indoors; except where “reasonably necessary” for work functions.
10 December 2020
The Department for Education has confirmed it had a workplace gathering to thank workers for their work throughout the pandemic. It says drinks and snacks had been bought by those that attended and no outdoors company or help workers had been invited.
14 December 2020
The Conservative Party has admitted that an “unauthorised gathering” occurred at its HQ in Westminster. It was held by the staff of the 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 talk to 2 individuals who attended the occasion.
16 December 2020
London moved into the highest tier of restrictions and Matt Hancock; who was the Health secretary an the time; stated it was necessary “everyone is cautious” forward of the Christmas Holidays
The Department for Transport apologised after confirming stories of a celebration in its places of work that day; calling it “inappropriate” and an “error of judgment” by workers.
17 December 2020
A leaving occasion was held on the Cabinet Office for the outgoing head of the civil service Covid taskforce – the staff answerable for drawing up coronavirus restrictions.
Kate Josephs; now chief govt of Sheffield City Council; apologised for the occasion; 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 initially denied a report by the Daily Mirror that an 'occasion' occurred in Downing Street.
However; a video obtained by ITV News confirmed the prime minister’s then-press secretary Allegra Stratton; joking about stories of an occasion; saying: “This fictional party was a business meeting and it was not socially distanced.”
12 April 2021
Lockdown restrictions had been eased in England; with pubs and eating places allowed to reopen with out of doors service only.
However; working from home continued to be mandated and socialising indoors with folks from different households was not allowed. Meeting others outside was restricted to groups of six folks or two households.
16 April 2021
Two events had been held by Downing Street workers at No 10; the evening before Prince Philip’s funeral.
One of the occasions was a leaving party for the PM’s then director of communications James Slack; who has apologised for the occasion and acknowledged it “should not have happened at the time that it did”.
Boris Johnson was not at either occasion.