The Opposition leader repeatedly denied breaking the laws in a televised statement on Monday afternoon as he faces pressure over the curry and beer gathering in Durham last year.
He accused the Conservatives accusing him of breaking lockdown rules of "trying to feed cynicism to get the public to believe all politicians are the same".
"I believe in honour, integrity and the principle that those who make the laws must follow them and I believe that politicians who undermine that principle, undermine trust in politics, undermine our democracy and undermine Britain," he said, from Labour's London headquarters.
"I'm absolutely clear that no laws were broken, they were followed at all times, I simply had something to eat while working late in the evening as any politician would do days before an election.
"But if the police decide to issue me with a fixed-penalty notice I would, of course, do the right thing and step down."
Having faced days of damaging headlines, Sir Keir was attempting to grab the initiative while putting pressure on Boris Johnson over his refusal to resign after being fined by police for a lockdown breach.
Sir Keir appeared to suggest he would even stand down if Durham Constabulary fine him for breaching the rules but decline to issue a fine retrospectively.
"If you've made a law you should respect the law and if you're found to be in breach of it you should step down," he told reporters.
Angela Rayner issued a statement making a similar commitment, insisting she was at the event "working in my capacity as deputy leader and that no rules were broken".
"Eating during a long day's work was not against the rules. We have a Prime Minister who has been found to have broken the rules, lied about it and then been fined. If I were issued with a fine, I would do the decent thing and step down," she said.