Atletico Madrid have warned Barcelona that "there will be consequences" if personal terms were agreed with Antoine Griezmann before July 1.
The French forward's release clause dropped from £180m to £107m at the start of this month, but Atletico suggested more than two weeks ago that they suspect personal terms may have been agreed earlier this year.
Club president Enrique Cerezo claimed that he did not know what would happen if it were to be proven that Barcelona agreed terms with Griezmann as early as March, but warned that they would have to face the "consequences".
According to the Daily Mail, Barcelona have gone to great lengths to avoid being seen as tapping up Griezmann, with their apparent interest in Neymar potentially being a smokescreen.
Atletico are believed to be unhappy with the situation, but their interest in Barcelona full-back Nelson Semedo could soften their stance.
Los Colchoneros have already had a busy week, having smashed their transfer record to sign 19-year-old Benfica starlet Joao Felix for £114m.
