The Premier League found a record 42 cases of COVID-19 among players and staff at various clubs over the past week.
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 they had closed down their training complex following positive COVID-19 tests in the first team.
Meanwhile, Manchester United's Premier League match at Brentford on Tuesday has been postponed due to a number of COVID-19 cases at the Old Trafford club, the league announced on Monday.
United had reported a "small number" of cases discovered among first-team staff and players on Sunday, with the squad who travelled to Norwich City for their Premier League clash on Saturday kept apart from those who had the virus.
"The decision (to postpone the Brentford-United game) was taken following guidance from medical advisers due to the exceptional circumstances of an ongoing COVID-19 outbreak within Manchester United's squad," the league said in a statement.
"First team operations at the club’s Carrington Training Complex were closed down today to help control the outbreak and minimise risk of further infection among players and staff."
Discussions took place with the Premier League as to whether it would be safe to fulfil Tuesday's fixture, with United deferring their travel to London until a decision had been made.
Brentford said they welcomed the decision to postpone Tuesday's match.
"We appreciate that there will be many fans who will be inconvenienced with the fixture postponement and we apologise for the disruption, which was out of our control," the club said.
"We respect the Premier League's decision and acknowledge that the health and safety of both clubs' players and staff must always come first.
"We wish everyone affected a speedy recovery and look forward to hosting Manchester United at the Brentford Community Stadium in the new year."