Fifa president Sepp Blatter has criticised the trend towards "modern slavery" in football and called on Manchester United to let Cristiano Ronaldo leave for Real Madrid if he so desires.

There has been an ongoing war of whispers emanating from both Real and United over Ronaldo for several weeks now and when asked about the protracted wrangle over the 23-year-old's future, Blatter insisted players should be allowed to leave with impunity when they want to move clubs.

"The important thing is, we should also protect the player," he told Sky News.

"If the player wants to play somewhere else, then a solution should be found because if he stays in a club where he does not feel comfortable to play then it's not good for the player and for the club.

"I'm always in favour of protecting the player and if the player, he wants to leave, let him leave."

Blatter believes the issue raises interesting questions about the way transfers and contracts are dealt with in the game.

"I think in football there's too much modern slavery in transferring players or buying players here and there, and putting them somewhere," he continued.

"We are trying now to intervene in such cases. The reaction to the Bosman law is to make long-lasting contacts in order to keep the players and then if he wants to leave, then there is only one solution, he has to pay his contract."

Blatter went on to dismiss the Premier League's much-criticised '39th step' proposals, insisting plans to play competitive matches overseas - either in the league or domestic cups - was doomed to fail.

He said: "The 39th game as presented will never happen. To my knowledge what they (the Premier League) want to do is perhaps to play some of the League Cup matches somewhere outside of England. That's the last information I got.

"They should just forget about that."