Barcelona say they will take "appropriate legal action" against Spanish newspaper El Mundo after it published details of forward Lionel Messi's contract, reports BBC.
Barcelona denied "any responsibility for the publication of this document".
Argentina international Messi, 33, joined Barcelona at 13 but handed in a transfer request in August.
His contract runs out in the summer and there is uncertainty if he will remain with the club.
Barcelona said in a statement that they "regret" its publication, adding: "FC Barcelona categorically denies any responsibility for the publication of this document, and will take appropriate legal action against the newspaper El Mundo, for any damage that may be caused as a result of this publication.
"FC Barcelona expresses its absolute support for Lionel Messi, especially in the face of any attempt to discredit his image, and to damage his relationship with the entity where he has worked to become the best player in the world and in football history."
Messi is Barcelona's record goalscorer and has won 10 La Liga titles, the Champions League four times and the Ballon d'Or - awarded to the world's best player - six times.
His controversial summer request to leave followed clashes with club president Josep Maria Bartomeu, who resigned in October.
Messi, who described Bartomeu's reign as "a disaster", said in December: "I don't know what I'm going to do yet."