(CNN) -- English Premier League champions Manchester United have posted a message on their official Web site, explaining that none of their players belong to social networks sites such as Twitter or Facebook.
The message advises users to treat any profiles in the names of one of their players with "extreme skepticism."
The club says this is because of the high numbers of people impersonating squad members online.
Wayne Rooney, Ryan Giggs and Darren Fletcher are believed to be among the players to have had genuine accounts on Twitter that have now been closed.
Twitter has caused a few upsets between players and clubs over the last year.
Sunderland striker Darren Bent caused controversy last summer after using the service to complain about the slow nature of transfer negotiations with former club Tottenham.
And as recently as last week, Liverpool striker Ryan Babel was forced to apologize to manager Rafael Benitez after complaining on his Twitter page about being left out of the squad to face Stoke City.