There has been a reported spike in positive coronavirus cases since Premier League players returned for pre-season training last weekend.
A total of 14 cases have been recorded at just 12 clubs, according to The Telegraph.
Premier League champions Liverpool recorded one positive case this week after embarking on their pre-season trip to Austria, while Manchester United, City, Chelsea and Wolves are not back yet following their European commitments this month.
As well as Jurgen Klopp’s men, Tottenham, Everton, Southampton, West Ham, Crystal Palace, Leicester, Newcastle, Sheffield United, Brighton and newly promoted Leeds are all back in training.
West Brom and Burnley will be back on Thursday, while it is not known when Aston Villa are returning to work, though Jack Grealish shared a snap of him on Instagram returning to England from his holiday this week.
Arsenal and Fulham are not back yet following the conclusion of the 2019-20 domestic campaign and their involvement in the FA Cup and play-off finals.
The figures are so far unofficial, with the Premier League only expected to begin regularly publishing test results when the season starts on September 12.
It is also claimed that clubs fear that cases could continue to rise after players were pictured flouting social distancing rules to party with rival stars on holiday over the past few weeks.
Tottenham’s Dele Alli was pictured with Leicester playmaker James Maddison in Ibiza along with Aston Villa captain Jack Grealish, while West Ham star Declan Rice holidayed with Chelsea’s Mason Mount on the Greek island of Mykonos and Blues midfielder Danny Drinkwater hung out with Foxes defender Ben Chilwell.
There is no suggestion that these players have tested positive for coronavirus, or that they have contributed to the spread of Covid-19 within Premier League clubs.
The number of positive cases is disproportionately high, compared with the 30 positive cases from around 35,000 tests carried out during Project Restart.
Players and staff members were following strict coronavirus guidelines but have since been released by clubs to enjoy time off in the close-season.
Players are understood to be among those who have tested positive, and not all are believed to have been asymptomatic.
All top-flight clubs will undergo testing in the next few days, and with more than three weeks to go until next season starts there is not expected to be too much disruption to the opening weekend.
However, it has fired a warning to players and staff to continue with the strict protocols that were in place for the final nine games of last season following the three-month absence of Premier League football during the coronavirus crisis.
All persons testing positive will now have to self-isolate for at least seven days.
Players are expected to be tested once a week when the season gets underway, rather than twice a week as they were from May to July.