Several Chelsea players have not returned for pre-season training after testing positive for COVID-19, according to reports in British media.

The reports did not name the players and Chelsea did not immediately respond to a Reuters request for comment.

Players who test positive have to self isolate for 10 days and return a negative coronavirus test before they can take part in the training.

Sheffield United, Brighton & Hove Albion and West Ham United were among the other Premier League clubs to report positive COVID-19 cases in recent weeks, the Times said.

Some 14 positive cases have been reported across at least 12 clubs in the English top flight.

Frank Lampard's side, who finished fourth last season, kicks off their campaign against Brighton on Sept. 14.