Speculation over the five-time Ballon d'Or winner's future has grown in recent months, with Ronaldo thought to be frustrated with Madrid's performances on the field and still hurt by last year's accusations of tax evasion by the Spanish authorities.
Former club Manchester United have long been linked with a return for the 32-year-old, who left Old Trafford for a then-world record £80m fee in 2009.
The Red Devils remain interested in the forward, but they will reportedly face competition for his signature from Ligue 1 side Paris Saint-Germain, who would be able to match Ronaldo's salary demands and ambition on the field.
Yahoo! Sport claims that club president Florentino Perez has told Ronaldo that he is free to look for a move away from the club amid reports that their relationship has badly deteriorated.
However, Madrid would still command a hefty transfer fee of at least £100m for the striker, who has scored 424 goals in just 419 appearances for the club across all competitions.