Sir Keir Starmer is to be investigated by police amid allegations he broke lockdown rules last year, after receipt of “significant new information”, Durham Constabulary said.
The Labour leader has come under pressure since footage emerged of him drinking a beer with colleagues in April 2021 in Durham during campaigning for the Hartlepool by-election.
He declined to answer questions from reporters in Carlisle over the investigation on Friday afternoon following confirmation from police.
Sir Keir this week said he wanted to focus on the cost-of-living crisis and not Conservative “mudslinging”.
At the time of the alleged gathering, non-essential retail and outdoor venues including pub gardens were open but social distancing rules, which included a ban on indoor mixing between households, remained in place.
A police statement on Friday afternoon said: “Earlier this year, Durham Constabulary carried out an assessment as to whether Covid-19 regulations had been breached at a gathering in Durham City on April 30 2021.
“At that time, it was concluded that no offence had been established and therefore no further action would be taken.
“Following the receipt of significant new information over recent days, Durham Constabulary has reviewed that position and now, following the conclusion of the pre-election period, we can confirm that an investigation into potential breaches of Covid-19 regulations relating to this gathering is now being conducted.”
Police did not elaborate on the new information.
A Labour Party spokesperson said: “We’re obviously happy to answer any questions there are and we remain clear that no rules were broken.”
Police previously said they did not believe “an offence has been established in relation to the legislation and guidance in place at that time and will therefore take no further action in relation to this matter”.
A spokesman this month said officers had “received a number of recent communications on this subject, which we are considering and will respond in due course”.
The force said its “general approach” was not to take “retrospective action”, including against former Downing Street chief adviser Dominic Cummings over his notorious trip to Barnard Castle to “test his eyesight” while the country was in lockdown.
The force has been asked to confirm whether this general approach remains in place.
The announcement came hours after the Labour leader celebrated taking three major London councils in the local elections.
He previously said no restaurants or pubs were open and the hotel he and colleagues were staying in at the time of the alleged offence did not serve food, so “if you didn’t get a takeaway then our team wasn’t eating that evening”.
Richard Holden, the Conservative MP for North West Durham, said police were doing “exactly the right thing” in investigating potential lockdown breaches involving Sir Keir.
Mr Holden, who had been pressing police to investigate after presenting new evidence, said: “It’s vital that the man who wants to be prime minister is held to the same standard as the Prime Minister and everybody else.”
Prime Minister Boris Johnson, his wife Carrie Johnson and Chancellor Rishi Sunak are among those to have already been fined for breaking Covid laws.