Athletic Bilbao are to consider taking action against Bayern Munich, according to the club's president Josu Urrutia.
The Bavarian club completed the signing of Bilbao's Javi Martinez last week for €40m, but Urrutia is unhappy that the 23-year-old travelled to Munich to undergo a medical before the money was paid to the club, and is looking at making a formal complaint to Uefa.
"Athletic are assessing whether they will take action against Bayern", he told a press conference on Monday.
"The player was under contract, during the protection period established by FIFA. The player underwent a medical with the German club without Athletic's permission. Whether we can or should denounce Bayern is being considered."
The Spain international made his debut for Bayern on Saturday, as the Bundesliga giants thrashed Stuttgart 6-1.