The Hammers claim to have made a club-record approach to sign midfielder William Carvalho during the recent transfer window - and reportedly have email evidence to prove it - but Sporting have denied that they ever received a formal bid for the Portugal international.
Sporting communications director Nuno Saraiva took to Facebook to brand West Ham co-chairman David Sullivan a "liar" and "parasite", prompting the Hammers to threaten legal action against the Portuguese outfit.
However, De Carvalho has now revealed that he will take the case to FIFA in order to protect his club's reputation.
"West Ham sent a lot of agents. They never sent a proposal. It's our feeling and we already informed West Ham's lawyers that we are going to make a complaint to FIFA," he told BBC Sport.
"I haven't received an email from West Ham. They should start thinking very hard about the legal issue. They are going to need to show FIFA that they sent. They need to show a lot of things.
"It is very bad for our image and he needs to understand he will pay for this. He has damaged the name of a club that is 111 years old."
Carvalho has made 155 appearances for Sporting since being promoted from the youth team in 2011.