The Labour leader's assertion came as a leaked memo indicated the takeaway in Durham had been planned as part of his itinerary for the day of campaigning in April 2021 and, no further work was scheduled after the dinner.
Allies of Sir Keir insisted that the curry was consumed between work events, meaning it was within the rules despite the ban on indoor socialising.
Police in Durham are looking into the event and Tory ministers lined up to accuse Sir Keir of hypocrisy after the Labour leader called for Boris Johnson to quit when he faced a Scotland Yard probe into No 10 parties.
At the time of the Durham gathering, non-essential retail and outdoor venues including pub gardens were open, but social distancing rules - which included a ban on indoor mixing between households - remained in place.
Sir Keir previously said no restaurants or pubs were open at the time of the alleged breach so "if you didn't get a takeaway then our team wasn't eating that evening".
Labour has also indicated that as Sir Keir was working the meal did not constitute a social event.
The leaked memo published by the Mail on Sunday showed that the dinner in Durham had been listed on the schedule for Sir Keir's day in April 2021 during the local election and Hartlepool by-election campaigns.
The document states that there would be a "dinner in Miners Hall" with City of Durham MP Mary Foy from 8.40pm to 10pm - and a note indicates a member of staff in Sir Keir's office was to arrange the takeaway curry from the Spice Lounge.
The only business listed after the dinner is for Sir Keir to walk back to his hotel - he has previously claimed he "paused for food" and continued working after the meal, saying "the idea that nobody works at 10 o'clock at night is absurd".
Separately, a source who was present told the Sunday Times: "It has been claimed that Starmer worked during the curry and then after the curry. None of those two things happened. He did not go back to work to the best of my knowledge."
The source said some of those present at the event with Sir Keir and deputy leader Angela Rayner were not working at all, and "were just there for a jolly".
A spokesman for the Leader of the Opposition's office said: "Keir was working, a takeaway was made available in the kitchen, and he ate between work demands. No rules were broken."
Timings for events frequently slip during a campaign and on the day of the curry the takeaway was late, a source said.
Sir Keir, a former director of public prosecutions, told reporters on Saturday: "As I have explained a number of times, I was working in the office, we stopped for something to eat.
"There was no party, no breach of rules, I am confident of that."
He said he would not resign and would lead Labour into the next general election.
Culture Secretary Nadine Dorries said: "It was a lie. No break in a work meeting. No pieces to camera, but a pre-arranged social dinner at which Rayner, Foy, and Starmer were present."
Foreign Office minister James Cleverly said: "Starmer claimed it was an impromptu curry. Turns out it was pre-planned.
"Starmer claimed nowhere served food. Turns out that loads of places did.
"(Angela) Rayner claimed she wasn't there. Turns out she was. Hypocrisy and dishonesty in equal measure."
Education Secretary Nadhim Zahawi told Times Radio "the public will be uncomfortable with the hypocrisy" adding: "He has tweeted himself saying that if you're under investigation, a criminal investigation, then you should resign."
In January, Sir Keir said the Prime Minister "needs to do the decent thing and resign" after he became embroiled in lockdown breach allegations.
Mr Johnson, his wife Carrie Johnson and Chancellor Rishi Sunak are among those to have already been fined for breaking Covid laws as part of the Met's investigation into No 10 parties.
The Sunday Mirror reported that more fines have been issued to people who attended a No 10 Christmas party on December 18, 2020 - the event which triggered aide Allegra Stratton's resignation after she was caught joking about it on camera at a mock press conference.
Mr Johnson is not believed to have attended that event.