Chelsea and Brazil midfielder Willian is taking legal action after insisting a document appearing to authorise a change of agents used a “forged” signature.

The 29-year-old has instructed his lawyers following the emergence of a letter which appears to show Denirobson Dias giving Renee Pinheiro Anunciacao permission to represent the Premier League player.

But Willian has rejected the validity of the letter, which he shared in an Instagram post on Thursday.

He wrote: “In the recent days I have been informed that a ‘letter of authorisation for player agent FIFA’, allegedly signed in my name has been circulated through the football market.

“In such letter, Mr Denirobson Dias would grant an authorisation to Mr Renee Pinheiro Anunciacao to represent me until August 15 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.”