Sir Keir Starmer is facing calls to resign after police launched an investigation into potential Covid law-breaking at the "beergate" event.

       The Labour leader drank beer and shared a curry with colleagues including Angela Rayner, the deputy leader, at an indoor gathering during the local election campaign last April.

       When Scotland Yard began an investigation in January into alleged lockdown-breaking parties in Downing Street, both Sir Keir and Ms Rayner repeatedly called for Boris Johnson to resign.

       Sir Keir tweeted a week after the announcement of the investigation: "Honesty and decency matter. After months of denials, the Prime Minister is now under criminal investigation for breaking his own lockdown laws. He needs to do the decent thing and resign."

       Ms Rayner tweeted on the day: "Boris Johnson's Downing Street is under police investigation, how on earth can he think he can stay on as Prime Minister?"

       On Friday afternoon, the same questions were being put to the Labour front bench about Sir Keir after Durham Constabulary issued a statement saying it would investigate the Labour gathering. The force had previously said it did not believe the Labour leader had breached Covid rules.

       Part of the statement read: "Following the receipt of significant new information over recent days, Durham Constabulary ... can confirm that an investigation into potential breaches of Covid-19 regulations relating to this gathering is now being conducted."

       Sir Keir insisted on Friday evening he was "confident no rules were broken", adding: "As I've explained a number of times, we were working in the office, we stopped for something to eat. No party, no breach of the rules.

       "The police have obviously got their job to do. We should let them get on it. But I'm confident that no rules were broken."

       Tory MPs have leapt on the development after months of being on the back foot as the Labour leadership hammered Mr Johnson over "partygate".

       Graham Stuart, the Conservative MP for Beverley and Holderness, tweeted: "It's the po-faced, self-regarding, smug naked hypocrisy of Starmer on 'partygate' that makes his comeuppance so satisfying.

       "Every speech, every interview, every script he used to try to force out the Prime Minister can now be deployed against him. Hoist with his own petard!"

       Richard Holden, the local Conservative MP who urged Durham police to reinvestigate the gathering, told The Telegraph: "Sir Keir Starmer has relentlessly attacked the Prime Minister over the last year.

       "Starmer's own words should guide his actions if he is an honourable man. If they don't, he will simply nail on the fact that he is nothing but a cheap political opportunist playing at politics with no interest in the principles he purports to hold."

       Sir Keir declined to answer questions about the investigation on Friday afternoon as he walked past reporters gathering for local election results coverage.

       The Labour leader had seen his party win three Tory councils in London – Wandsworth, Westminster and Barnet – but struggle to make the breakthroughs desired in the rest of England.

       A Labour spokesman said of the police investigation: "We're obviously happy to answer any questions there are, and we remain clear that no rules were broken."

       Labour sources are expected to rebut suggestions of a direct parallel between the one event over which Sir Keir is being investigated and the dozen of government events into which the Metropolitan Police is looking.

       Yvette Cooper, the shadow home secretary, declined to say that Sir Keir should resign if he is fined, despite the Labour leader's previous calls for Mr Johnson and Rishi Sunak, the Chancellor, to do just that.

       During an interview on BBC Radio Four's World at One moments after news of the investigation broke on Friday, Ms Cooper said: "I think that the party has always been very clear about this. It was a work event in the middle of a local election campaign."

       She added that it would not be appropriate for her to comment on speculation about the case given her brief.

       There were scores of examples of Sir Keir and Ms Rayner calling for Mr Johnson to go after the Met launched its investigation but before he had been fined.

       Both the Prime Minister and Mr Sunak were issued with fixed penalty notices over an event to mark Mr Johnson's birthday during lockdown.

       At Prime Minister's Questions on Jan 26, Sir Keir said: "I think the Prime Minister said yes, he agrees the [ministerial] code does apply to him. Therefore, if he misled Parliament, he must resign."

       In an article for the Politico website published that day, Ms Rayner wrote: "It feels almost beyond belief that the Metropolitan Police are now investigating evidence of 'the most serious and flagrant type of breach' in Downing Street itself.

       "What a reflection on the man who holds the very highest office in our country. Yet still he feels he can hang on? A complete disgrace."

       The Durham investigation continues, with no decisions announced on whether any attendees at the Labour event will be fined.