A party source says “documentary evidence” shows Sir Keir Starmer’s team were working until 01:00 BST.
Durham police are investigating reports of a gathering where the Labour leader ate a curry and drank a beer at an MP’s office during an election visit.
Sir Keir has pledged to resign if he is fined over the allegations.
The Labour Party is confident it can prove that the gathering in April last year was permitted under rules that allowed work events, saying that those attending were working late on the campaign for the Hartlepool by-election.
The Guardian said the party has compiled time-stamped logs from WhatsApp chats, documents and video edits which show work continued after the takeaway food was delivered.
A party source said: “We have been totally clear that no rules were broken. We will provide documentary evidence that people were working before and after stopping to have food.”
In a statement on Monday, Sir Keir repeatedly said no laws had been broken, as he tried to contrast his position with that of Boris Johnson, who refused to resign after being fined by the Metropolitan Police over his birthday celebration at No 10 Downing Street in June 2020.
Having repeatedly called for the prime minister and Chancellor Rishi Sunak to quit after it was found they had broken the law, the Labour leader has come under increasing pressure to make his own position clear, with the Conservatives accusing him of “rank double standards”.
When asked if the Labour leader had now assumed a position of moral authority over the government, Policing Minister Kit Malthouse said: “I don’t particularly.”
Mr Malthouse refused to say whether Sir Keir should resign if he was fined, saying “that’s a matter for him” and Durham police should be given “the space and time to do their job”.
Labour’s deputy leader Angela Rayner, who was also present at the meal, said she too would resign if she was fined.
But Culture minister Chris Philp accused Sir Keir of “attempting to pressure the police into clearing him” by announcing that a fine would mean the end of his leadership.
Mr Philp said it was “deeply inappropriate”.
Durham police announced on Friday that it was launching an inquiry into the Labour event, having previously said it did not believe any offence had been committed.
It is thought the investigation could last up to eight weeks.
A video of the gathering on 30 April 2021, which took place in the constituency office of City of Durham MP May Foy, shows Sir Keir drinking a bottle of beer while colleagues in the background are eating.
Under the Covid laws in England at the time, indoor gatherings were banned – but there was an exception for “work purposes”.
Sir Keir said in his statement that “no laws were broken” and he “simply had something to eat while working late in the evening”.
He suggested even his critics did not really believe the allegation against him, but were simply trying “to get the public to believe all politicians are the same”.
“I’m here to say they are not – I believe in honour, integrity and the principle that those who make the laws should follow them,” he added.
“If the police decide to issue me with a fixed-penalty notice, I would of course do the right thing and step down.”
The Labour leader faced questions about whether he had left himself a loophole if police suggest he may have broken Covid rules, but do not issue a fine.
Durham Police previously came to a similar conclusion when they investigated the PM’s former adviser Dominic Cummings for driving to Barnard Castle during the first Covid lockdown.
Sir Keir said: “The penalty for a Covid breach is a fixed penalty notice, that’s a matter of law, and I’ve set out what the position is in relation to that.”