FIFA president Sepp Blatter has compared long-term contracts between soccer players and their clubs as a form of "modern slavery".

Speaking to Britain's Sky Sports, Blatter said that too many clubs were trying to tie players up in long-term contracts to prevent them moving to other teams.

"I think in football there is too much modern slavery, transferring players or buying players here and there, and putting them somewhere," Blatter said. "And we are trying now to intervene in such cases."

FIFA's revised regulations on the status and transfer of players allow players to terminate contracts prematurely if they have played in less than 10 percent of their club's matches over the course of a season.

More controversially, an article of the regulations also gives players the option to walk out on existing contracts after two or three years, depending on their age, provided they pay compensation to the club they leave.

Asked specifically about Manchester United's [Images] efforts to prevent their Portuguese forward Cristiano Ronaldo [Images] leaving to join Real Madrid [Images], Blatter said the English side should let him go.

"If the player wants to play somewhere else, then a solution should be found," he said. "Because if he stays in a club where he does not feel comfortable to play then it's no good for the player and for the club."

Although Ronaldo has already spent more than three years at United, he could still face a lengthy playing ban if he tried to buy out his contract with the English and European champions.

The transfer regulations state that players wishing to terminate contracts should give notice within 15 days of the end of their club season or face possible disciplinary measures.

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