Sir Keir Starmer promised to step down as leader of the Labour party if he is fined by police for breaking lockdown rules.
The opposition chief is being investigated by officers over claims he breached Covid restrictions with staff last April.
In what has been dubbed “Beergate”, Sir Keir was pictured drinking with party workers while they enjoyed takeaway curry.
The Labour boss says he was working late into the night and says no rules were broken.
However, he would come under pressure to resign if he is fined since he has demanded Boris Johnson step down for breaking Covid rules.
Announcing his intention to quit if given a fixed penalty notice would allow Sir Keir to continue saying the prime minister should go.
“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 on Monday afternoon.
“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.”
It comes as Nicola Sturgeon claimed “Beergate” was an attempt to distract from the prime minister’s own lockdown breaches.
She said: “There is a massive operation underway on the part of the Conservatives to divert attention from Boris Johnson.
“And not just Boris Johnson’s single breaking of the rules, but what appears to have been a serial breaching of the rules and, of course, Boris Johnson’s inability to be straight with the House of Commons.”
Downing Street has refused to confirm whether they believed the Labour leader should stand down if he is given a fine.
Labour’s shadow health secretary Wes Streeting claimed the Tories had been “slinging mud” at his boss for weeks.
Key allies of Sir Keir insisted last week he had done nothing wrong when it emerged police in Durham were investigating.
Officers initially snubbed calls to probe the gathering, but later reversed course and said new details had come to light.