LONDON (AFP) - FIFA president Sepp Blatter has urged Manchester United to stop treating Cristiano Ronaldo like a 'modern-day slave' and let the Portuguese player join Real Madrid if he wants to.

Ronaldo, a boyhood Real fan, has been linked with a move to the Spanish capital since the end of last season even though he scored a hugely impressive 42 goals in all competitions last term as United won both the Premier League and Champions League trophies.

Wading into the protracted wrangle over the 23-year-old's future, Blatter said that the current practice of tying players to long contracts amounts to "modern slavery".

And he urged Manchester United and Real Madrid "to sit together" if Ronaldo wants a move to the Spanish club.

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

"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 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."

United issued a swift rebuke to Blatter's outburst, insisting that all their players were happy to sign contracts with the club and knew the consequences of entering into such an agreement.

A United spokesperson said: "All our players - like at other clubs - enter into their contracts after an open and free negotiation.

"Most of whom do after taking advice from a FIFA-registered agent.

"Many do so on a number of occasions and enjoy long and successful stays at Old Trafford."

UEFA's communications director William Gaillard, who serves as special advisor to European football governing body president Michel Platini, was also unimpressed by Blatter's outburst.

"It would be useful to remind people that slaves in all of the slavery systems never earned a wage," said Gaillard.

In contrast, Gaillard fears the Bosman ruling, which allows for free transfers at the end of contracts, has given players the upper hand and contributed to spiralling wages.

"It seems that both clubs and players are trying to negotiate an exit before the player is free," Gaillard said.

"It is a consequence of the Bosman ruling - there is nothing we can do about that.

"It is obvious that today players have a lot more power than they did 20 years ago, undoubtedly, and agents have a lot more power than they did 20 years ago.

"It is true that salaries are spiralling out of control. Many clubs have pointed that out."

Meanwhile, 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."