David Beckham, Zinedine Zidane, Luis Figo, Raul and Ronaldo, together with the Spanish club, are concerned that seven on-line bookies have used the names and pictures of the club, the players and club strips in ads without consent, reports the Guardian newspaper.
Infringement suits have therefore been filed against the firms – which include Ladbrokes, William Hill and Sporting Exchange - in Germany, France and Belgium.
Bookmakers are reportedly unsure as to the specifics of the allegations.
"There is no indication from Real Madrid as to when we may have used unauthorised images and what they have said is couched in such general terms that it is difficult to know what we will be defending ourselves against," Graham Sharp, a spokesman for Ladbrokes, told the Guardian.
In the UK, the law of passing off can sometimes be used to prevent a celebrity's image being used overtly to promote a commercial product. The closest the UK courts have come to recognising a "celebrity right" was racing driver Eddie Irvine's success against talkSPORT in the High Court in March 2002, although the decision was consistent with existing principles. It simply recognised the commercial value of personal endorsements.
In Irvine's case, a promotional brochure was sent to less than 1,000 people advertising radio station Talk Radio, with a photo that had been doctored to show Irvine holding a radio bearing a Talk Radio logo, instead of a mobile phone, which he was holding in the original photo.