Fighting for his political life, the prime minister was asked whether he could resign over the scandal, and replied: "We'll have to see what [the internal inquiry] says.”
The PM’s former chief of staff Dominic Cummings has said Mr Johnson dismissed his warnings that the “bring your own booze” event was against the law – a claim apparently corroborated by other sources.
Six Tory MPs have already called for Mr Johnson to step down, with the governing party tanking in the polls and now at least 10 points behind Labour.
Asked if he had lied to the House of Commons over the parties, the prime minister said: "No. I want to begin by repeating my apologies to everybody for the misjudgments that I've made, that we may have made in No 10 and beyond, whether in Downing Street or throughout the pandemic.
"Nobody told me that what we were doing was against the rules, that the event in question was something that ... was not a work event, and as I said in the House of Commons when I went out into that garden I thought that I was attending a work event."
Mr Johnson said he "could not imagine why on earth it would have gone ahead, or why it would've been allowed to go ahead" if he had been told it was not a "work event".
"I do humbly apologise to people for misjudgments that were made but that is the very, very best of my recollection about this event, that's what I've said to the inquiry," he said.
"I carry full responsibility for what took place but nobody told me, I'm absolutely categorical, nobody said to me this is an event that is against the rules."
The prime minister also insisted that he only saw the "bring your own booze" invite that his principal private secretary Martin Reynolds sent to more than 100 staff "the other day... when it emerged".
"Nobody warned me that it was against the rules, I am absolutely categorical, because I would remember that," he added.
Angela Rayner, Labour's deputy leader, reiterated her party's call for Mr Johnson to step down.
“Boris Johnson clearly knows it’s the end of the road," she said. “He’s the Prime Minister, he set the rules, he didn’t need anyone to tell him that the party he attended broke them. If he had any respect for the British public, he would do the decent thing and resign.”
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies