Boris Johnson made a statement in front of the House of Commons at Prime Minister’s Questions today, on Wednesday, January 12.
He confirmed he did briefly attend the event, before returning to his office.
Mr Johnson has faced furious demands from MPS, who have insisted he come clean about his attendance of the party.
On Monday, ITV news shared a leaked email, containing an invite for the party which was sent by Prime Minister's Principal Private Secretary Martin Reynolds to over a hundred employees.
Here is Boris Johnson’s statement in full:
"I want to apologise. I know that millions of people across this country have made extraordinary sacrifices over the last 18 months.
"I know the anguish they have been through - unable to mourn their relatives, unable to live their lives as they want or to do the things they love.
"I know the rage they feel with me and with the Government I lead when they think 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 there were things we simply did not get right and I must take responsibility.
"No 10 is a big department with a garden as an extension of the office which has been in constant use because of the role of fresh air in stopping the virus.
"When I went into that garden just after six on May 20, 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.
"With hindsight I should have sent everyone back inside. I should have found some other way to thank them.
"I should have recognised that even if it could be said technically to fall within the guidance, there are millions and millions of people who simply would not see it that way, people who have suffered terribly, people who were forbidden for meeting loved ones at all inside or outside, and to them and to this House I offer my heartfelt apologies.
"All I ask is that Sue Gray be allowed to complete her inquiry into that day and several others so that the full facts can be established."