Premier League has been hit by 14 new cases of coronavirus since teams returned to training, reports claim.
According to The Telegraph, the new cases have been recorded at 12 clubs who began preparations for next season last weekend.
The figures are unofficial, but the Premier League are only expected to regularly publish test results when the new season starts on September 12.
Club officials are said to be concerned cases could continue to rise after players were pictured partying on holiday with pals from rival clubs over the past few weeks.
Tottenham midfielder Dele Alli was snapped with James Maddison and Jack Grealish in Ibiza.
And Chelsea transfer target Declan Rice holidayed with life-long friend Mason Mount on the Greek island of Mykonos, where Tammy Abraham, Fikayo Tomori and Christian Pulisic were also pictured.
Blues midfielder Danny Drinkwater also hung out with his former Leicester colleague Ben Chilwell.
There is no suggestion any of these players have tested positive for coronavirus, or that they contributed to the spread of Covid-19 within the Premier League.
League champions Liverpool recorded one positive case this week after flying to Austria to being their pre-season.
Manchester United, City, Chelsea and Wolves have not yet returned following their European fixtures this month.
But Tottenham, Everton, Southampton, West Ham, Crystal Palace, Leicester, Newcastle, Sheffield United and newly-promoted Leeds have all begun training again.
West Brom – returning to the top flight – and Burnley confirmed they will be back on Thursday, while it is uncertain when Aston Villa and Brighton will follow.
Arsenal and Fulham have enjoyed extended breaks after their involvement in the FA Cup and play-off finals.
After the Premier League restarted in June, players and staff members were following strict coronavirus guidelines and tested twice a week.
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But they have since been released by clubs to enjoy time off during the short gap until the new term gets underway.
All top-flight clubs will undergo testing in the next few days and anyone to have tested positive will have to self-isolate for at least seven days.
Players are expected to be tested only once a week when the new season starts.
