The Premier League has returned its highest number of positive coronavirus results in a single round of testing this season, with 18 new cases recorded.
The League confirmed that, between 21-27 December, 1,479 players and club staff were tested and the 18 individuals who have the virus will now self-isolate for a period of 10 days.
A number of those cases are at Manchester City after an outbreak there saw their game with Everton called off at four hours' notice on Monday.
The previous highest number of cases in one round this season was 16, during the week of 9-15 November.
A Premier League statement read: "The Premier League can today confirm that between Monday, 21 December and Sunday, 27 December, 1,479 players and club staff were tested for Covid-19. Of these, there were 18 new positive tests.
"Players or club staff who have tested positive will self-isolate for a period of 10 days.
"The Premier League is providing this aggregated information for the purposes of competition integrity and transparency. No specific details as to clubs or individuals will be provided by the League and results will be made public after each round of testing."
Sheffield United revealed "a number" of their players are self-isolating after testing positive for Covid-19.
The Blades, who face Burnley in the Premier League on Tuesday evening, said in a statement: "Sheffield United can confirm the club recorded a number of positive coronavirus tests after the latest round of testing.
"Due to medical confidentiality, the club will not be revealing the names of those who have been affected.
"The individuals who have tested positive have now entered into a period of self-isolation in accordance with Premier League and Government guidelines."
Despite the news, the Blades have confirmed that Tuesday night's game will go ahead as planned.
Meanwhile, Rochdale's club doctor Wes Tensel has called for the EFL to see the "bigger picture" and halt the current campaign due to rising cases of coronavirus.
Dale are the latest club to suffer an outbreak of Covid-19 in their camp over Christmas, with their next two games called off.
Millwall, Rotherham, Ipswich, Sunderland, Hull, Doncaster, Peterborough, Portsmouth and Morecambe have also been hit over the last week.
Tensel, who is also a practising GP, says there are consequences for football clubs beyond the pitch and thinks a postponement of the season would be a sensible move.
"This is my opinion as a doctor. Some people will disagree, some won’t, this is just my opinion based on what I have seen working in the NHS and at a football club," he told the PA news agency.
"As a club we try to minimise the spread of Covid as much as possible, but we have heard about this new strain which is more transmissible, does it make sense for clubs to be travelling across tiers when other people are unable to see their relatives in another tier over Christmas?
"People could only see their loved ones outside in a park, but a whole coach-load of footballers can go up and down the country as they wish."
