The Portugal international signed a contract extension last year to keep him at Old Trafford until 2012.
Speaking in January, Manchester United chief executive David Gill told Football Focus: "There's no chance of us selling him, no way, whatever the bid.
"Cristiano signed a new contract last season to 2012 and we are not a selling club."
And selling the club's leading scorer would not sit well with last week's statement from Gill outlining plans to further improve the squad over the summer.
"We will certainly be in a position where, if we can improve it, we will," Gill told MUTV.
However, Real Madrid have made no secret of their desire to sign Ronaldo, with their coach Bernd Schuster describing Ronaldo as "the best player in the world" earlier this year.
On Wednesday, Miguel Angel Arroyo, Real Madrid's general director, said: "We have the solvency and the financial liquidity to be able to take on a signing of the magnitude of Cristiano Ronaldo.
"It would be economically beneficial due to the merchandising that would arise from it."
Real have signed big names from United in recent seasons, with David Beckham and Ruud van Nistelrooy both moving from Old Trafford to the Bernabeu.
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