Manchester United received up to nine positive cases from lateral flow tests at their Carrington training complex on Thursday.
It is understood up to five players and four staff members returned positive tests that were carried out at Carrington on Thursday.
United are awaiting the results of PCR tests for the players and staff members who tested positive but have cancelled their scheduled friendly at Preston North End on Saturday.
Manager Ole Gunnar Solskjaer's coaching staff was forced to isolate in February following a positive case and, although United have previously had isolated cases of Covid-19 on the playing staff, they had managed to avoid any form of an outbreak.
United's training complex is cleaned on a regular basis and the club say they are following Premier League protocols. The academy and women's squads gather in separate buildings to the first-team at Carrington and will be able to continue training.
United Under-23s' behind-closed-doors friendly at Hull City on Saturday is still scheduled to go ahead.
A statement released by United on Thursday night said: "Maintaining Covid security is a priority for us. Following routine testing of the first team training group today, we have identified a small number of suspected positive cases. This has led to those people isolating, pending further tests.
"As a precautionary measure based on covid protocols, we have taken the difficult decision that we will not now be able to play the friendly match against Preston North End this Saturday.
"We regret the disruption to Preston and disappointment caused to fans. Any Manchester United fans who have purchased tickets for the game will be automatically refunded.
"At this stage, we do not expect further disruption around our forthcoming matches, but we will continue to follow Premier League protocols in this regard."
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