Cristiano Ronaldo's agent has issued a thinly-veiled threat that the 20-year-old may leave Manchester United unless offered an improved contract.

The Portuguese international is only two years into a five-year contract at Old Trafford, but agent Jorge Mendes told the Daily Express: "In three years Cristiano Ronaldo will be the best player in the world. I've got no doubts about that.

"I think he will stay but I also have to do the best for my client."

Ronaldo was a £12.24million signing from Sporting Lisbon in 2003, and has since established himself as a key member of Sir Alex Ferguson's side.

"He signed the existing contract at the age of 18, after a handful of games for Sporting, but now he has developed into a star," said Mendes.

"Cristiano has huge ambition. At Sporting, there were lots of clubs after him, all the big names from Italy and Spain.

"But when I sat down with Alex Ferguson I knew it was the right choice because he told me that Cristiano would play one week and then have a rest, then play, then rest and that he would not be burned out."