On a visit to a construction site in West London, the Prime Minister said: "We are concerned about Leicester, we are concerned about any local outbreak.
In response to the flare-up in cases, current lockdown restrictions could remain in place in Leicester for two weeks past July 4 to control the outbreak, according to the city's mayor.
It would mean bars, pubs and restaurants in the city do not open in line with the rest of England on Saturday.