Sevilla have confirmed they will sue Atletico Madrid over the transfer of forward Vitolo, the club announced on Monday.
Vitolo signed with Atletico this summer, though he will play for Las Palmas until January because the Madrid club is still serving a transfer ban imposed by FIFA.
Atleti captured Vitolo from Sevilla in July in a deal that saw the Spain international trigger his own €37.5 million release clause.
However, Sevilla say Vitolo agreed to a contract extension with their club two days earlier and the deal could not be unilaterally breached by the player.
Sevilla immediately said its board of directors planned legal action over the situation, and after receiving the backing of an analysis conducted by legal experts, will now push forward with their plans to sue all parties involved, including Atletico, Las Palmas and Vitolo.
Vitolo, a Las Palmas academy product, joined Sevilla from the club for €3.2m in 2013.
He appeared in 43 matches over all competitions last season, scoring six goals and providing 10 assists.
Information from The Associated Press was used in this report.