Sir Keir Starmer will be investigated by police over claims he broke lockdown rules.
The Labour leader was pictured drinking beer with staff in Durham last April when Covid restrictions were still in place.
The opposition chief has repeatedly denied he breached curbs in place and said he had been working late.
Sir Keir claimed he had ordered takeaway curry along with staff due to most restaurants being closed.
Durham Police had initially said they would not probe Sir Keir over the apparent lockdown breach.
But officers confirmed an investigation into the Labour boss will now be held.
Partygate quit demands
Sir Keir has repeatedly demanded Boris Johnson resign for breaking lockdown rules.
The Conservative leader was fined for a birthday party held for him in June 2020 while the nation stayed at home.
He could face further fines as police continue to investigate Downing Street gatherings held while restrictions were in place.
The prime minister claimed that a Downing Street garden party held in May 2020 had been a “work event”.
However, Sir Keir could also now face significant pressure if he is given a fixed penalty notice by police.
Labour shadow minister Emily Thornberry said on Sky News she was confident her party leader would be cleared by police.
A spokesperson for 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.”
A Labour spokesperson said: “We’re obviously happy to answer any questions there are and we remain clear that no rules were broken.”