Wireless's promotor Live Nation said that sellers of fake tickets were "preying on young music fans" and engaging in criminal activity.
It issued a firm warning to fans during the festival, saying: "Some unfortunate fans that have purchased 'tickets' via third party websites such as Gumtree and eBay have been refused entry to the festival as their tickets are counterfeit.
"Unfortunately we are unable to let you in to the festival as it is sold out and we advise you to make any future purchases through an official outlet such as www.ticketmaster.co.uk."
Those with fraudulent tickets are advised to contact Action Fraud.
Around 65,000 fans went to the three-day festival to watch performances by artistes including Drake and Nicki Minaj.
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