Manchester United have been put on high alert with reports claiming Cristiano Ronaldo will leave Real Madrid at the end of this season.

Ronaldo's return to Old Trafford has been a common transfer window theme, and the big story could finally become a reality, if the Portuguese does, indeed, decide his time at the Bernabeu is up.

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While Zinedine Zidane insisted he would stick to the "BBC" trident of Gareth Bale, Karim Benzema and Cristiano Ronaldo, the 30-year-old, Real's all-time leading scorer, is still likely to leave at the end of the season.

PSG are the favourites to sign Ronaldo, if he does leave Real Madrid, but Manchester United are also likely to make a bid to bring one of their greatest ever players back. Ronaldo fuelled rumours of a move to PSG earlier this season, when he went to Laurent Blanc and whispered something, before giving the French club's chairman a friendly pat on the back.

However, the Portuguese has also admitted in the past he would love to come back to Manchester United. The English Premier League side have also been heavily linked with a move for Bale next summer, but the Welshman is likely to remain at the Bernabeu.

Real Madrid president Florentino Perez is a big fan of Bale, and wants to see the former Tottenham man become the marquee player of the club. Zidane, when asked about Bale's role at the club, also said he wants the Welshman to succeed at the Bernabeu.

"Out of Bale and Ronaldo, Ronaldo is the player expected to leave the club," a Real Madrid source told the Telegraph. "Bale is still seen as the future, whereas Ronaldo's best years may now be behind him.

"PSG are desperate to sign him and would be favourites at the moment, but United will no doubt try."