Sir Keir Starmer has pledged to quit as leader of the UK’s Labour party if he is fined for breaking coronavirus restrictions by police who are investigating an evening meal in April last year.

The opposition leader is under investigation by Durham police over allegations that restrictions were broken during a campaigning event which he attended. Starmer was photographed with a bottle of beer.

“If I am given a fixed penalty notice I will do the right thing and step down,” Starmer said on Monday. “This is about integrity.”

Starmer drew a distinction with prime minister Boris Johnson, who has rejected calls to resign after being fined by police for breaching Covid lockdown rules at an event in Downing Street.

“I’m very different to the prime minister in this regard,” Starmer said. “He and others in his party wants us to believe that we are all the same . . . I’m here to show you that is not the case.”

Those who have spoken to Starmer about the police probe say he is confident he will not be fined.

“All the advice he is getting is that he didn’t break rules and that the police will confirm that,” one ally said.