The announcement was made at 7.45pm on Thursday evening and comes as a huge blow to North End fans who were due to return to Deepdale after an absence of 511 days.
A statement released by United 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."
The United game was North End's only home friendly of the pre-season programme.
That programme will now close with Friday night's visit to Wigan Athletic which will take on even more significance as Frankie McAvoy shapes the side for the Championship opener against Hull City.
North End said: "Details of how PNE supporters can claim a refund for their tickets will be announced on the morning of Monday 2nd August.
"We would like to apologise to all supporters for any inconvenience caused, however, the Covid security of both clubs is paramount with the 2021/22 Championship season just eight days away."
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