Boris Johnson has admitted he attended a garden party at Downing Street during lockdown in May 2020.
The British prime minister has apologised for attending the event.
He said he thought it was a work event but "must take responsibility" for what happened.
Details of the party have emerged in the British media in recent days.
It emerged 100 staff members were invited to the gathering and told to bring their own booze.
Around 30 people ultimately attended the event.
Under COVID rules at the time, the public was restricted to meeting one other person outside.
After previously refusing to answer whether he attended the party, Boris Johnson made a statement in the House of Commons today.
He said: “I know millions of people across this country have made extraordinary sacrifices.
“I know the rage they feel with me and with the government I lead when they think in Downing Street itself the rules aren’t being properly followed.
“There are things we simply did not get right, and I must take responsibility.”
Mr Johnson acknowledged that he went into the garden for 25 minutes during the party in May 2020 - saying he believed it was a “work event”.
He said: “With hindsight, I should have sent everyone back inside and found some other way to thank them.”
He said he will come back to the House of Commons and make a statement after an internal investigation into a series of gatherings at Downing Street is concluded.
Opposition parties called for Mr Johnson to resign in the wake of the revelations.
Labour leader Keir Starmer called Boris Johnson's statement "the pathetic spectacle" of a man who had run out of road.
He said: "Is he now going to do the decent thing and resign?
"Why does he still think the rules don't apply to him?"
He added that Mr Johnson's apology was "pretty worthless".
Mr Johnson said Mr Starmer 'must wait' until the findings of Sue Gray's investigation is completed to find out the full facts of what happened.
He added that he "bitterly regrets" what unfolded at the event.