The Portuguese Real Madrid star insists that the allegation – made in Guillem Balague’s book, MESSI – is untrue, and is threatening legal action.
"News is circulating that I allegedly made offending remarks regarding Lionel Messi," he wrote on his official Facebook page.
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"This is absolutely false and I have assured my lawyer takes action to sue those responsible.
"I have the utmost respect for all my professional colleagues, and Messi is obviously no exception."
That does not match the claim made by Spanish football journalist Balague, who writes, in a book serialised by the Daily Telegraph: "Ronaldo, perhaps as a symptom of the immaturity that marks so many footballers, thinks it necessary to put on a brave face in front of his team-mates, not be scared of Messi and to rise to the challenge.
"All very macho; all very false. And that is why, according to some Real Madrid players, CR7 has a nickname for him: "mother******"; and if he sees someone from the club speaking to Leo, he also ends up being baptised "mother******"."
Ronaldo and Messi are set to face each other on Tuesday when Portugal and Argentina play in a friendly at Old Trafford.