Keir Starmer is under investigation by police over allegations he broke coronavirus lockdown laws while have a beer and a takeaway with staff in a constituency office.
The Labour leader was pictured drinking a bottle of beer with campaigners days before the crunch by-election in Hartlepool last year.
Durham Police have been under intense pressure from the Tories to investigate the gathering in April 2021, when indoor meet-ups were banned.
Back in February, the force said it would not be taking action against Mr Starmer but it said it had received "significant new information" in recent days.
Mr Starmer said: "As I have 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 obviously have got their job to do – we should let them get on with it.
Labour leader Sir Keir Starmer speaks to supporters in Barnet
"But I'm confident that no rules were broken."
Labour has pointed to an exemption in lockdown laws for "work purposes". Political campaigning was allowed ahead of the local elections, which took place the same day as the by-election.
But the news of police investigation poses a major problem for Mr Starmer who has been vocal in his criticism of Boris Johnson for breaking his own lockdown laws.
The Prime Minister, his wife Carrie and Chancellor Rishi Sunak have all received fines from the Metropolitan Police over a surprise birthday bash for Mr Johnson in June 2020.
Mr Starmer has repeatedly rejected Tory claims he had broken the rules, which he branded as "mudslinging".
He previously said: “I was in a constituency office days before the election, we were very busy, we were working, at some time some food turned up.
“We stopped, we had our food and then we carried on working. That is not a breach of the rules, it’s not a party and it’s no comparison to the Prime Minister."
He added: "It was perfectly lawful to meet for work, which is what we were doing."
Asked where he draws the line, the Labour leader replied: “The line is drawn in the rules – and the Prime Minister broke the rules.”
Labour clinched victory in Barnet in local government elections
He previously said no restaurants or pubs were open at the time and the hotel he and his colleagues were staying in did not serve food, so "if you didn't get a takeaway then our team wasn't eating that evening".
Today, Durham Police 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.”
The police did not elaborate on the nature of the new information.
The force previously said their "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.
A Labour Party spokeswoman said: "We're obviously happy to answer any questions there are and we remain clear that no rules were broken."
Barrister Adam Wagner, an expert on Covid laws, said the event would likely be viewed as similar to colleagues eating together at work – compared to the PM's birthday bash which was a social event.
He said: "There is a difference in law between a pre-arranged social event (birthday party) and a meal whilst campaigning when there is nowhere else to get food.
"There is no prospect of showing a pre-arranged birthday party is reasonably necessary for work – it is a purely social event."
Meanwhile, Mr Starmer has been out and about congratulating new Labour councillors after declaring the local elections have got his party "back on track".
He said: "We have turned a massive corner in the Labour Party here. We're winning in London, we're winning north and south."