The Premier League found a record 42 cases of Covid-19 among players and staff at various clubs over the past week, it said on Monday just hours after Manchester United's fixture against Brentford was thrown into doubt due to an outbreak at the club.
The 42 positive tests from over 3,800 individuals is more than the tally from the previous seven weeks combined and a record in the English top-flight in a single week.
"The League has reverted to its Emergency Measures, and will increase the frequency of both Lateral Flow and PCR Covid-19 testing of players and staff," it said in a statement.
United said it had closed down it's training complex following positive Covid-19 tests in the first team.
United was scheduled to play at Brentford on Tuesday but the club said it had approached the Premier League to discuss whether the match should go ahead.
The Old Trafford club did not say how many individuals had tested positive for the novel coronavirus.
"Individuals who tested positive are isolating per Premier League protocols," the club said in a statement.
"Given the cancellation of training and disruption to the squad, and with the health of players and staff the priority, the club is in discussion with the Premier League whether it is safe for Tuesday's fixture against Brentford to continue.
"Team and staff travel to London will be deferred pending the result of that discussion."
Aston Villa manager Steven Gerrard and Brighton & Hove Albion coach Graham Potter also said they had players who had tested positive for Covid-19.
"We re-tested this morning and we have got a couple of staff and a couple of players missing for the foreseeable future," Gerrard said.
Potter said Brighton had "three or four" cases. "Now we're just checking and making sure it's at that number."
Last week Tottenham Hotspur had two matches called off due to a Covid-19 outbreak at the club affecting players and staff -- it's the final Europa Conference League group game against Rennes and the weekend's league trip to Brighton.
While the Brighton game has been postponed, UEFA said the Conference League tie will not be played due to a lack of available days on the calendar, with Spurs risking an exit from the competition if it is forced to forfeit.