Ronaldo is expected to undergo surgery on an ankle early next week after he was seen by a specialist on Friday and may miss the start of the new season.

United, meanwhile, have been infuriated by a report on Friday claiming that the 23-year-old will sign for Real within a matter of days.

A club spokesman said: "It is utter nonsense and totally laughable. Cristiano Ronaldo is not for sale."

Despite Ronaldo's reported desire to move and a huge fee of around £70million being on offer, United chiefs believe selling the player would cause huge disruption to the club.

Apart from the effect on the team's playing style and formation, it would also send a signal that United are in some way a lesser team than Real.

The Glazer family bought United because they viewed it as the biggest club in the world so they cannot afford to been seen as weaker than Real Madrid, one of their biggest European rivals, or in thrall to their star player.

The message chief executive David Gill has given to the Glazers is that if they stand firm on Ronaldo and neither sell him nor give him another big new contract then it will strike a major blow against player-power and have long-term future benefits for the club.

There is no real financial incentive to the Glazers either - the club's profits are already covering their debt repayments easily so there is no requirement to raise new capital.

If they considered selling him, United manager Sir Alex Ferguson would argue that he would need all the £70million fee as transfer funds to replace his most valuable player.

Confirmation of Ronaldo's operation comes after a specialist echoed the view of medical staff from both United and the Portuguese FA that the ankle injury needs surgery.

He could be out for up to six weeks, and could be out for the start of the season.

United leave for their tour of South Africa on July 18 but Ronaldo was not going on the trip anyway.