Tetchy former PM Boris Johnson was accused of employing a "deflection mechanism" by a Tory MP as he tried to convince a probe he hadn't deliberately misled Parliament.
He refused to name officials who he claims told him rules hadn't been broken in Downing Street before he told MPs guidance had been "followed completely".
Mr Johnson is fighting for his political future as the Commons Privileges Committee probes whether he committed contempt of Parliament. He could be suspended if the allegation is upheld.
The grim-faced MP for Uxbridge and South Ruislip faced bruising questioning from the cross-party panel.
Tory MP Alberto Costa told him: "Why did you tell the House of Commons that you'd received repeated assurances that no rules had been broken when you knew that wasn't the case because you knew what the rules were.
"You were at gatherings that breached the rules and the breaches of the rules would have been obvious to you at the time.
"Some might say your reliance on the purported assurances you received are a deflection mechanism to prevent having to answer questions about your knowledge of the gatherings."
Mr Johnson dismissed this as a "ridiculous" assessment.
The shamed ex-PM was shown footage of himself telling Parliament that all Covid guidance was followed - despite this later transpiring to be untrue.
In one clip from December 2021 he told Labour leader Keir Starmer at PMQs: "What I can tell the right honourable gentleman is that all guidance was followed completely in No 10."
In a rambling defence Mr Johnson claimed a boozy leaving do at No10 during Covid restrictions was "necessary" after being challenged by Tory Sir Bernard Jenkin in an icy exchange.
Sir Bernard pointed out that "no work" was being done in photos from November 2020, which show the ex-Prime Minister raising a glass with colleagues.
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In a bruising session an uncomfortable Mr Johnson was shown photos of himself just over a year earlier at a leaving do for two senior officials, which appeared to show people failing to social distance.
Sir Bernard said: "The photographs don't show any work being done, why didn't it occur to you that it may not have been reasonably necessary for work purposes?"
Sir Bernard then pointed out that holding leaving dos "wasn't acceptable" for everyone else.
Flustered Mr Johnson said: "What I was doing was thanking staff for their contribution, I believe that was my job."
He said the leaving drinks were "necessary" because two people - Lee Cain and Dominic Cummings - were leaving in "potentially acrimonious circumstances" and it was important for him to provide reassurance.
Mr Johnson is claimed to have told staff that it was "the least social distanced event in the country". When asked about this, the former PM did not deny the remark, but said: "I think it unlikely that I would have said those words."
He also said he had "no sense" that a birthday celebration - which included his then-fiancee Carrie and interior designer Lulu Lytle - was against the rules.
In his opening address Mr Johnson said: "I'm here to say to you hand on heard that i did not lie to the House when those statements were made in good faith."
He said he initially dismissed the outrage about gatherings in No10 as a "Daily Mirror try-on" after this newspaper exposed the rulebreaking events.
In a lengthy statement he also lashed out at former chief advisor Dominic Cummings - who questioned Mr Johnson's claim that he didn't know rules were broken - telling the committee: "He has every motive to lie."
He claimed it was "nonsense" that he would have known he was breaking rules, and claimed it was "staggeringly implausible" that an official photographer would have been asked to photo an "illicit" event.
He continued: "If we had an event that was unauthorised, why would we have it on Zoom?"
"If it was obvious that these events were contrary to the rules it must have been unquestionably obvious to some of the most senior officials in the country and it must have been obvious to others in the building, including the current Prime Minister," he said.
He said that accusing him of lying meant civil servants and politicians were being accused of lying.
Mr Johnson admitted that staff often came within one-and-two metres of each other, saying there was no "electrified forcefield around every human being".
But he said the "cramped" surroundings in No10 made it impossible to maintain perfect social distancing.
Harriet Harman, who is chairing the cross-party committee, said misleading the House is a "matter of great importance".
She said: "If what minsters tell us isn't the truth, we can't do our job. Our democracy depends on trust."
Ms Harman said the committee had collected direct first-hand evidence from witnesses, and was not relying on the Sue Gray report.
Bombshell new Partygate evidence was published this morning showing the No10 scramble to defend Mr Johnson and the warnings given to the ex-PM about lockdown boozing.
The Commons Privileges Committee disclosed 110 pages of messages and statements from key players in Downing Street ahead of its explosive showdown with Mr Johnson later today.
The slippery former PM is arguing that he didn't mislead Parliament intentionally when he denied rules had broken at lockdown bashes because he had received assurances from aides.
But one account shows Mr Johnson was told by senior aide Martin Reynolds not to say that all Covid guidance was followed in No10 at PMQs following the Mirror's report that Partygate booze-ups had happened during the pandemic.
Civil service chief Simon Case flatly denied to the committee that he had ever given these assurances and said he wasn't aware that anyone else did.
The bumper dossier also contains messages showing the then-No10 spin chief Jack Doyle arguing the Mirror would "get bored" of the story if they offered a robust defence to our exclusive story in December 2021.
And one No10 official gave evidence that Mr Johnson "had the opportunity to shut down" lockdown gatherings in Downing Street but "joined in".
Rivka Gottlieb, spokesperson for Covid-19 Bereaved Families for Justice, said: "Today was a new low for Boris Johnson. It's clear he lied when he said to our faces that he'd done 'all he could' to protect our loved ones, he lied again when he said the rules hadn't been broken in Number 10, and he's lying now when he denies that was the case.
"He claims it was 'his job' to say goodbye to colleagues, that he 'would have needed an electric fence' around him to stick to the rules, and that social distancing only applied 'when possible'.
"Did any of this apply when we couldn't be with our loved ones for weeks as they suffered alone in care homes and hospitals, or even be there to hold their hands in their dying moments?
"Bereaved families found it painful to watch him pull his usual tricks of deflection, self-pity and blaming everyone but himself. The fact that it appears he didn't fully understand the rules he was setting and communicating to the nation is especially galling.
"He isn't fit for public office and if had any respect he'd resign as an MP and quietly reflect on the pain and suffering he has inflicted on so many."
