Atletico Madrid chief executive Miguel Angel Gil Marin said his club are considering reporting Barcelona for an illegal approach for Antoine Griezmann.
Barcelona's board made a public attempt to persuade Griezmann to make a €100 million switch to the Camp Nou this summer, but the France international used a television documentary last week to announce his decision to sign a new deal that ties him to Atletico through 2022-23.
"We have proof of the contacts," Gil Marin told Cadena Ser. "There is no doubt that Barcelona have interfered in talks between Atletico and a player who is under contract. Evidently, that is illegal, and we will weigh up whether or not to report Barcelona to the [Spanish football] federation."
Atletico first grew angry at their Barca counterparts in December and then considered informing FIFA of a breach of transfer regulations -- something that could have serious consequences considering Barca have been sanctioned for breaking youth transfer rules in the past.
Amid reports that Barcelona were not willing to match the €23m-a-year salary offered by Atletico, Gil Marin said it had been the chance to lead a "project" that had convinced the 27-year-old to stay.
"You do not convince people just with money," Gil Marin said. "We sold him on being important in an ambitious project [at Atletico]. ... At Barcelona, he was going to be a soldier for Leo Messi. It is true that [Messi] is the best player in the world and it is normal that anyone who goes there is to help him build his own story. Here, it is the other way around. Here, Antoine can build his history around the history of Atletico Madrid."
Barcelona vice-president Jordi Mestre said Tuesday at the presentation of new technical secretary Eric Abidal that the club followed all appropriate protocols in their pursuit of Griezmann.
"As a club, we respect any player's decision to decide their own future," Mestre said. "We had officially informed Atletico Madrid of our interest in the player. Barca wants players who are committed and determined to triumph here, above and beyond any other circumstance."
Griezmann also referred to the "project" when speaking to Atletico's website about his decision.
"Since I arrived at the club, we have grown so much," Griezmann said. "We have a brand new stadium that has been praised by everybody and which allows us to empower the growth of recent years. I love the project we are creating with great players. I thank the big effort the club are making. Miguel Angel Gil told me his ideas for the coming seasons and I got really excited. The project is incredible."
Atletico have also agreed to sign Monaco winger Thomas Lemar in a reported €60m transfer and tie down another France international, Lucas Hernandez, to a long-term contract.
Hernandez pointed to Lemar's arrival as an example of the efforts the club have made to compete at the highest level.
"The club have grown so much in the last years," he told the club website. "We have a very important project. I am really enthusiastic about what we are preparing for the next season. Huge efforts are being made so that Atletico Madrid continue to grow and it's something that all players are excited about. We have great players who stay for the project and others like Lemar who are attracted by those great possibilities that the club shows."