Opposition Leader Keir Starmer described the apology as “pathetic” and “worthless”.
“I know that millions of people across this country have made extraordinary sacrifices over the last 18 months,” Mr Johnson told Prime Minister’s Questions on Wednesday.
“I know the anguish that they have been through, unable to mourn their relatives, unable to live their lives as they want or to do the things they love.
“And I know the rage they feel with me and with the government I lead when they think that in Downing Street itself, the rules are not being properly followed by the people who make the rules.
“And though I cannot anticipate the conclusions of the current inquiry, I have learned enough to know that there were things we simply did not get right and I must take responsibility.”
Mr Johnson insisted the party was within lockdown guidelines in place at the time, which allowed some exemptions for work and funerals, but admitted many Britons would not see it that way.
“When I went into that garden, just after six on the 20th of May 2020, to thank groups of staff before going back into my office 25 minutes later to continue working, I believed implicitly that this was a work event,” Mr Johnson said.
“But, Mr Speaker, with hindsight, I should have sent everyone back inside.”
Mr Starmer described the statement as “the pathetic spectacle of a man who has run out of road” after “months of deceit and deception” and a string of opposition lawmakers accused the PM of lawbreaking, lying and debasing his office.
“His defence that he didn’t realise he was at a party is so ridiculous that it’s actually offensive to the British public,” the Labour leader said, in Parliament.
“He’s finally ready to admit what everyone knew, that when the whole country was locked down, he was hosting boozy parties in Downing Street.
“Is he now going to do the decent thing and resign?”
Mr Johnson said he rejected the resignation calls and repeatedly asked his opponents to wait for an investigation by senior civil servant Sue Gray into several alleged parties by government staff.
The email from the Prime Minister’s private secretary, Martin Reynolds, was sent to dozens of people and urged attendees to “bring your own booze.”
The event was scheduled for May 20, 2020 — the same day the government at a televised news conference reminded people they could only meet up with one person outside their household.
During Britain’s first lockdown, which began in March 2020 and lasted for more than two months, gatherings were banned with a few exceptions, including work and funerals.
Millions of people were cut off from friends and family, and even barred from visiting dying relatives in hospitals.
On the day of the garden party, 268 people with the coronavirus died in Britain, according to official figures, bringing total deaths to more than 36,000. The total now stands at more than 150,000, the highest toll in Europe after Russia.
There has been widespread anger at claims the government flouted the rules it had imposed on the rest of the country by holding garden parties, Christmas get-togethers and office quiz nights in Downing Street, which is both the prime minister’s home and his office.
The Conservatives have an 80-seat majority in the House of Commons.
But many members of his own party are increasingly concerned about Mr Johnson’s judgement and leadership.