Keir Starmer has launched a dramatic fight-back over 'Beergate' by vowing to quit if he is fined by Durham Police.
In a high stakes move, the Labour leader tied his future to the fate of the police investigation launched by detectives last week.
The former director of public prosecutions insisted he was "very clear" that no rules were broken last April when up to 15 staff ate curry and drank beer while campaigning.
But 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."
His deputy Angela Rayner, who was also at the event, said she would "do the decent thing" and resign if she received a fixed penalty notice.
The dramatic move piles pressure on the Prime Minister who has refused to quit despite being fined by the Met Police for breaking lockdown laws.
Hannah Brady, spokesperson for Covid-19 Bereaved Families for Justice, said: "This is the right decision by Keir Starmer and in contrast to Boris Johnson, shows integrity, decency and respect to the bereaved."
In an interview with the Mirror last night, Mr Starmer was determined that no rules had been broken.
"I know what we did and I've obviously looked at the regulations. We were working. We ate," he said.
"Five days before an election when you've got a team with you, that is part of the working day."
He added: "As a matter of principle for me, it's very important we have honour, integrity and accountability in politics.
"It's about who I am, the integrity that I have and the integrity I want to see in politics.
"The Tories want to drag everyone down, to run this argument that everyone is the same because the Prime Minister won't do the right thing, even though he's broken the law.
"I think it's very important to make clear that not all politicians are the same."
However, Mr Starmer indicated he would only stand down if he was actually fined by Durham Police.
"The penalty for breaching Covid rules is a fixed penalty notice," he said.
"We know that from everything that's happened in Downing Street".
It gives him a loophole in the event officers conclude the gathering was a minor breach but don't issue a FPN.
Mr Starmer suggested he would not challenge any fine in court: "Police need to go through investigation. If they issue a FPN what I will do is the right thing and step down."
The Labour leader and his aides spent hours locked in discussions about whether he would pledge to quit.
The Mirror understands the force only contacted his office yesterday.
But he claimed the move was inevitable ever since Durham Police announced it would investigate.
"This is a matter of principle and it was always a decision I was going to take. It was always going to be this way," he said.
The former top lawyer said it was "quite clear" the Tories were "putting pressure" on police to look again at the gathering in the run up to polling day.
But he added that it was "laughable" for Government insiders to suggest he was now doing the same after Tory MPs called for him to quit.
"If they don't understand even now what integrity means then I'm sure they're further down the slippery slope than any of us even realised," he said.
The Labour leader had faced mounting allegations over the campaign when indoor gatherings were banned, but with exemptions for work purposes.
Mr Starmer insisted he went back to work after stopping for a quick bite to eat - which was within the rules.
"I was working throughout. I had a busy schedule the next day. When you're on the campaign trail as leader of the Labour party you're working all of the time.
"This is not something special on that night, that's what happens when you're on the campaign trail, you're working round the clock."
Labour is prepared to hand Durham police a dossier of evidence, including time-stamped logs of WhatsApp chats, documents and video edits, to show Mr Starmer’s team worked beyond 1am.
He claimed the operational notice leaked at the weekend showed it was "absolutely clear" he had been working.
"As for who was ordering the curry, that wasn't really my responsibility. I'm working, food arrives.
"Any politician that has been on the road knows that somebody in the team has responsibility for making sure there's a meal."
He denied reports that local MP Mary Foy and some of her staff were drinking heavily at the event.
"No. My team were working. They were all working in the office," he said.
He claimed a failure to reveal that Ms Rayner was also present had been a "genuine mistake" by his office.
But Mr Starmer said he did not regret calling for Mr Johnson to quit before he had been fined.
"He had flatly denied at the despatch box that anything at all happened," he said.
"He hasn't been straight with Parliament. that was amongst the reasons why I called for him to go.
"He should quit. I don't think he will. I don't think he's got any integrity."
And he insisted he would continue to hold the Government to account over the Partygate scandal.
"Absolutely. I'm holding myself to the high standards that I say Boris Johnson should be held to."
But he admitted he was frustrated that the beergate row was overshadowing Labour's local election results and the focus on the cost of living crisis.
"I feel frustrated that a very good set of results for us has not had the attention it should have done because of this smear.
"I feel anger on behalf of my teams who worked so hard to get those election results."
He added: "The number one issue wherever you go in the UK is 'how can I pay my bills?'.
"The Government has been an abject failure, and made a bad situation worse by taxing people in the middle of a cost of living crisis."
Mr Starmer admitted his own family - like millions of others - had to make "agonising decisions" over lockdown.
"The idea that I'd just casually break the rules having observed them throughout so carefully is not true."
He added: "My family understands first hand what so many families went through.
"It was very difficult because my wife's mum died suddenly in awful circumstances.
"When the lockdown came my wife had to sit on the pavement outside her dad's house because she wouldn't break the rules.
"Then she tossed and turned at night because she felt guilty because she hadn't done the right thing by her dad."
He denied he was "claiming any special case" as countless other families had to make difficult decisions, often involving an elderly relative or a child, to stick to the rules.
It came as a YouGov poll found that 46% of voters thought the Labour leader should resign if he was fined for lockdown breaches.
