Atletico Madrid have accused Antoine Griezmann and Barcelona of "disrespect" and say the La Liga champions talked to their forward in March.
Atletico say they have rejected Barcelona's offer and ordered Frenchman Griezmann to report for pre-season.
In a statement, the Madrid club expressed their "strongest repulsion for the behaviour of both, especially Barcelona".
Griezmann, 28, said in May he would leave Atletico this summer.
The France international has scored 94 La Liga goals in 180 games for Atletico and is widely expected to move to Barcelona.
He has a buyout clause in his contract of 120m euros but Atletico say their La Liga rivals want to postpone payment and were talking to him months ago.
"Obviously, the response from Atletico was negative. Understand that both Barcelona and the player have disrespected Atletico and all its fans," the Atletico statement added.
"On 14 May, Antoine Griezmann informed the club of his decision to leave the club at the end of the season.
"In the days following that meeting, Atletico learned that Barcelona and the player had reached an agreement last March, specifically in the days after the second leg of our Champions League tie against Juventus, and had been negotiating the terms of the agreement since mid-February.
"[This is] something we consider violates protected periods of negotiation with players and alters the basic rules that govern the integrity of all sports competition, besides being a huge loss to our club and its millions of fans."
'We never talk about other players'
Earlier on Friday, Barca president Josep Bartomeu said the club had made contact with Griezmann and that their former forward Neymar "wants to leave Paris St-Germain".
Brazil international Neymar left Barcelona for PSG in 2017 for a world record fee of 222m euros (£200m) and has been linked with a return.
Bartomeu also said he knows where 19-year-old Netherlands defender Matthijs de Ligt, strongly linked with a move away from Ajax, will be playing next season.
Barcelona have already signed De Ligt's international team-mate Frenkie de Jong from the Dutch champions.
"The first contact with Griezmann was yesterday," said Bartomeu on Friday. "There is an interest, that's why we got together.
"We never talk about players from other teams.
"We know Neymar wants to leave PSG but we know that PSG do not want Neymar to leave.
"I said long ago I knew where De Ligt would play. I still know it and I cannot say more."
During the unveiling of De Jong, Bartomeu was asked about Atletico's statement, adding: "We are introducing De Jong and I will talk about it at another time."