T In The Park fans are being urged not to buy 2009 tickets from a Hungarian-based website, BBC News reports.
Organisers have warned festival-goers that www.tinthepark2009.com has not been given any passes for next July's event and fans should not give out their credit card details to the site.
This comes with the news that bosses have applied to increase the festival's capacity by 5,000 to 85,000, with councillors due to vote on the proposal tomorrow (12 December).
Speaking about the Hungarian-based website, a T In The Park spokesperson said: "This site is 100% unofficial and has not been given an allocation of festival tickets to sell.
"The event is taking legal action and urges T in the Parkers to under no circumstances buy tickets from this outlet."
T In The Park 2009 will take place at Balado Airfield, Kinross-shire from 10-12 July.
Early rumours suggest that Coldplay, Franz Ferdinand and Glasvegas may play the festival, which was awarded with the Best Line-Up award at the UK Festival Awards.