Willian is taking legal action after being the victim of an alleged fraud over his Chelsea future.
The Brazilian has been linked with a move away from Stamford Bridge with Manchester United and Barcelona both keen.
He is yet to return to pre-season training but has taken to social media to reveal he has been the victim of identity theft.
A letter purportedly sent from a representative claiming to act on behalf of Willian, emerged appearing to grant another representative permission to discuss a move for the player.
The letter, with FIFA logos on it, claims to have been signed by the Chelsea star - but he has denied any involvement and confirmed legal action has been taken.
"In the recent days I have been informed that a “letter of authorization for player agent FIFA”, allegedly signed in my name has been circulated through the football market," the Brazilian's Instagram post read on Thursday.
"In such letter, Mr. Denirobson Dias would grant an authorization to Mr. Renee Pinheiro Anunciação to represent me until 15 August 2018, throughout football clubs in the United Kingdom and France.
"I would like to clarify that I never signed such letter, the content of which I completely deny.
"Finally, since there seem to be clear evidence that my signature has been forged, or anyway misused, I hereby inform that my lawyers are already instructed to proceed with all appropriate legal measures before any competent courts or authorities.
"I hope I have clarified the matter and any possible misunderstanding."
Willian has been linked with a £65m move to Manchester United to reunite with Jose Mourinho.
But last week, he insisted he was happy at Chelsea and was not considering his future.
He was involved in a bust-up with former boss Antonio Conte at the end of last term but will be given a fresh start under Maurizio Sarri.
Chelsea are yet to comment.