It is thought Durham Constabulary are to look again at whether the Labour leader a gathering in April 2021 was against the law.
Sir Keir was filmed drinking a bottle of beer with colleagues in the Durham office of Mary Foy, the local Labour MP, when mixing indoors with those from another household was banned.
He has insisted he followed the rules and the incident was a work event.
Durham police originally rejected claims to investigate whether rules were broken by the Labour leader.
However, there has been growing pressure on the force to reconsider after Prime Minister Boris Johnson was fined for an event inside No10 in which he was presented with a birthday cake.
Conservative MPs say there is little difference between the two events and that if Mr Johnson was fined, the Labour leader should also face investigation.
In the face of growing demands from Tory MPs, The Telegraph reports the police are to now launch a formal investigation.
More to follow…