United have said they have “identified a small number of suspected positive cases” and that those affected after self-isolating in line with government guidance.
With under three weeks until the start of the new Premier League season, United say they do not expect further disruption around their forthcoming pre-season matches.
A Manchester United statement read: “Maintaining Covid security is a priority for Manchester United.
“Following routine testing of the first-team training group today (Thursday), 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 North End and disappointment caused to fans.
“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.”