Sir Keir insisted no rules were broken while he had a takeaway in a constituency office while working on the election campaign last year.
He repeatedly refused to apologise and was branded a “hypocrite” by a caller during his LBC Radio phone-in show.
The Labour leader has demanded the resignation of Boris Johnson over No. 10 parties held during 2020 when England was locked down.
Sir Keir was pictured in the office of City of Durham MP Mary Foy on April 30, 2021 in the run-up to May’s local elections and the Hartlepool parliamentary by-election.
He said: “The picture of me was in a constituency office up in the North East. It was I think, three or four days before the May elections, so we’re really busy. I was with my team going across the country from place to place.
“We’re in the office, working in the office and we stopped for a takeaway, and then we carried on working and that is the long and the short of it.
“There was no breach of the rules. There was no party. And there was absolutely no comparison with the Prime Minister.”
Sir Keir said no restaurants or pubs were open and the hotel he and his colleagues were staying in did not serve food, so “if you didn’t get a takeaway, then our team wasn’t eating that evening”.
Asked if he was prepared to apologise, Sir Keir said: “We didn’t break any rules, we were working in the office and we stopped for a takeaway.”
He added: “We did nothing wrong.”
The LBC caller who asked about the incident dismissed Sir Keir’s response as “like listening to Boris Johnson without the harrumphing”, adding: “You should apologise and you are a hypocrite for not.”
The Labour leader was asked whether he would prefer to fight a damaged Mr Johnson at the next election, rather than Rishi Sunak or Liz Truss – the frontrunners to succeed the Prime Minister.
Sir Keir said: “I will take on whoever is leading, I don’t really care.
“I think that it’s in the national interest that Boris Johnson goes now … put party politics to one side, he’s lost all authority and that matters, whatever party you are in.
“We’re still in the pandemic and it’s very important that people behave in the way that we need them to behave, but he has lost the authority to ask people to do so.”
On other domestic and foreign issues, Sir Keir said the Prime Minister was “too weak to lead” and Britain was “paralysed” as a result.
Education secretary Nadhim Zahawi called for Sir Keir to apologise over the beer-drinking incident.
He told BBC Breakfast: “I think people expect, you know, very high standards from their leaders, and I think that’s only right.”
The accusations levelled against Sir Keir come with senior official Sue Gray looking into a litany of possible events, including a “bring your own booze” garden party during the first coronavirus lockdown that Mr Johnson has admitted he attended – although he insists he understood it to be a “work event”.