The Â£80million transfer fee for Cristiano Ronaldo has been described as "excessive" by UEFA president Michel Platini.
Real Madrid's offer for the Portuguese winger, which has been accepted by Manchester United, follows soon after the Spanish club's Â£56million swoop for Kaka.
Platini said in a statement: "These excessive transfers are happening almost every day.
"These transfers represent a serious challenge to the idea of fair play and the concept of financial balance in our competitions.
"UEFA are working hard with clubs to establish a new set of rules as soon as is possible to clean up the system and give it a more solid and more transparent base.
"That is our top priority today."
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