Durham police said they would begin the investigation into the potential lockdown breaches after receiving “significant new information”.
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.
Sir Keir this week said he wanted to focus on the cost-of-living crisis and not Conservative “mudslinging”.
13 pictures as Sunderland fans celebrate promotion in the city centre
13 fantastic pictures from the STACK at Seaburn as Sunderland clinch play-off final win
'Everyone loved him' - Family pay tribute after funeral of motorcycle crash victim Anthony 'Buster' Kirtley
13 of your fan pictures ahead of Sunderland's League One play-off final at Wembley
Tributes paid to ‘massive’ Sunderland fan Tony Staplin after tragic death in County Durham crash
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 spokesman for Durham Constabulary 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 Party spokesperson said: “We’re obviously happy to answer any questions there are and we remain clear that no rules were broken.”
Writing on Twitter, he said: “Only a matter of time. The law should be applied fairly and consistently across the country.”
The announcement comes hours after the Labour leader celebrated taking three major London councils following the local elections.
He previously said that 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”.
Prime Minister Boris Johnson, his wife Carrie Johnson, and Chancellor Rishi Sunak are among those to have already been fined for breaking Covid laws.
Tory MP Richard Holden has said Durham Police are doing “exactly the right thing” in investigating potential lockdown breaches involving Sir Keir Starmer.
Mr Holden, the North West Durham MP 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.
“For the last fortnight Starmer and Labour HQ have obfuscated, denied, lied, and attempted to smear in an attempt to shield Starmer and other senior Labour figures from reasonable questions which they have been unwilling to answer.
“I am glad that they will now have to answer them.”
Sir Keir Starmer declined to answer questions from reporters in Carlisle over the Durham Police investigation.
After leaving via a side entrance out of a hotel in the city, where he had been celebrating Labour’s win of control over the newly-created Cumberland Council, Sir Keir was asked for his reaction, but only said “thank you” to reporters.
He then left to board a train, accompanied by advisers and police officers.