The allegation was made in a book, Messi, by respected Spanish journalist Guillem Balague, who said that according to some Real Madrid players, Ronaldo calls the Barcelona star "motherf----r" behind his back and gives the same name to any other Madrid player seen to talking to him.
The Portuguese superstar, via his official Facebook page, has refuted his claim and says he is instructing his lawyers to take action.
Superstars and rivals: Cristiano Ronaldo and Lionel Messi. Photo: Getty Images
Balague says the two rivals are not friends, but are polite, if somewhat cool, to each other in public.