Chelsea midfielder Willian says he has instructed his lawyers to act after a "forged" document was circulated on the "football market".
The letter appears to show the 29-year-old's signature authorising a change of agent to represent him.
However, Willian claimed he "never signed the letter" in an Instagram post on Thursday.
Willian has been linked with moves to Manchester United, Real Madrid and Barcelona this summer.
In the post, Willian said he had been made aware of a document that "has been circulated through the football market".
"I would like to clarify that I never signed such letter, the content of which I completely deny," he added.
And he alleged there was "clear evidence that my signature has been forged, or anyway misused", which had prompted him to instruct his lawyers "to proceed with all appropriate legal measures before any competent courts or authorities".
The midfielder, who has spent five years at Chelsea, was due to report to pre-season training with the Blues last Friday, after representing Brazil at the World Cup in Russia, but he failed to arrive.
New manager Maurizio Sarri says he is not happy about the "strange" situation, and he will seek immediate talks with Willian.