West Ham have reported Sunderland to the Premier League for an illegal approach for striker Demba Ba.
The Hammers believe the Black Cats have contacted Ba, who is still under contract at Upton Park, about a move north, without their consent.
Sunderland have denied that they have approached the Senegal international.
The Premier League has said it has received a complaint from West Ham and will investigate the matter to see "if there is a case to answer".
A Sunderland spokesman said: "Sunderland AFC is fully aware of the Premier League regulations pertaining to player recruitment.
"If we are interested in any player currently under contract with another club, we will speak directly to that club, in line with the regulations."
Premier League regulations state that a club cannot approach a contracted player at another club, directly or indirectly, without written consent.
Ba is among a group of West Ham players whose futures look to lie away from Upton Park, following the club's relegation from the Premier League.
The 26-year-old, who scored seven goals for West Ham, has been linked to a number of Premier League clubs after impressing since his move from German club Hoffenheim in January.