The move could mean Boris Johnson is in an awkward position if he is fined again, as Political Correspondent Libby Weiner reports
Labour leader Sir Keir Starmer and his deputy Angela Rayner have both promised to resign if they are found by police to have broken coronavirus laws last year, however the pair insist no rules were broken.
"I'm absolutely clear that no laws were broken, they were followed at all times," Sir Keir said in a speech at Labour HQ, but 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 is under investigation by Durham Police over a gathering in the Durham office of Labour MP Mary Foy in April 2021, when most indoor socialising was banned.
The force previously cleared him of wrongdoing after viewing a photograph which showed him drinking a beer with other staff present, but last week said it had launched a formal criminal investigation after “significant new information” came to light.
Sir Keir has always insisted no rules were broken but some newspapers have uncovered details which they claim suggest otherwise.
The Labour leader, explaining the gathering, says staff who had been working on the Hartlepool byelection campaign on the evening of April 30 paused briefly to eat a takeaway before resuming their work.
But some newspapers are reporting that up to 30 people were present at the gathering, some who hadn't been working on the campaign, and a number were drunk - suggesting more work was not on the agenda.
He held a press conference on Monday to address allegations against him, saying he "simply had something to eat whilst working late in the evening, as any politician would do days before an election".
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He claimed those accusing him don't actually believe he broke the rules and are "trying to feed cynicism to get the public to believe all politicians are the same".
Speaking at party headquarters in London, he said: "I believe in honour, integrity and the principle that those who make the rules must follow them."
After pledging to step down if issued with a fixed-penalty notice, Sir Keir added: "This matters. It matters because the British public deserve politicians who think the rules apply to them.
"They deserve politicians who hold themselves to the highest standards. And they deserve politicians who put the country first rather than themselves. They will always, always get that from me."
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Ms Rayner, who admitted attending the gathering, said: "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."
Sir Keir avoided questions from reporters on Monday morning as he was asked whether he'd resign if police issue him a fine, after he cancelled a speech scheduled for today late on Sunday.
He's repeatedly called on Boris Johnson to resign for breaking his own coronavirus rules by celebrating his 56th birthday in June 2020, after he, his wife Carrie and Chancellor Rishi Sunak were fined £50 for attending the gathering.
Critics had labelled him a hypocrite after refusing to promise his resignation if police decide he broke the rules but spent time with his teams devising a response on Monday morning.
Universities Minister Michelle Donelan said “it does smack of sheer hypocrisy” when asked about allegations that Sir Keir broke coronavirus lockdown rules.
Asked whether the Labour leader should step down if he was given a fine, she said: “That’s a decision for Keir, not for me. My constituents are saying that this whole thing smacks of sheer hypocrisy given the relentless focus he has placed on partygate.
“I think this is a decision for him, he’s going to have to search his soul after making this a top priority over the last few months at the expense of key issues like rising cost of living, etc, but look this is a decision for him."
Until now the Labour leader said he would not resign and would lead Labour into the next general election.
He 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."