LaLiga has called for Kylian Mbappe's contract at Paris Saint-Germain to be ripped up, demanding French government officials act and league chiefs take a closer look at the capital club's spending.
France striker Mbappe signed a new three-year deal with PSG in May, turning down Real Madrid who had been courting him for the past year.
The terms of the contract have not been officially disclosed, but Mbappe is widely reported to have received a staggering signing-on fee as well as extraordinary wages. Figures of €100million for signing and €50m per season have been suggested.
Lawyer Juan Branco, representing LaLiga, told a news conference in France on Friday that Spanish league chiefs are ready to go to court to challenge PSG's finances.
Ligue 1 champions PSG have strenuously denied being in breach of financial fair play regulations, but the Spanish authorities have expressed doubts about the legitimacy of their vast recent outlay on players. PSG have been owned by the Doha-based Qatar Sports Investments since 2011.
LaLiga wants the Ligue de Football Professionnel (LFP) and the DNCG, the body that oversees football club finances in France, to examine in detail whether the Mbappe deal should have been given the green light.
"We are going to request the repeal of the approval of Mbappe's contract from the minister of sports, because that is the supervisory authority of sports administration," Branco said.
"Subsequently, we will appeal to the LFP so that it acts via its legal commission the DNCG in order to operate a monitoring report on the accounts of PSG. This is a legal step which will allow us to establish whether Mbappe's contract is within the economic parameters which are imposed by the regulations of the DNCG and UEFA financial fair play."
PSG signed Lionel Messi last August after Barcelona ran out of money to hand their captain a new contract, and also acquired former Real Madrid skipper Sergio Ramos at the end of his Santiago Bernabeu stay.
Real Madrid were optimistic about landing Mbappe, but hopes in Spain that he would move to LaLiga were dashed.
Branco said LaLiga was prepared to take action via the administrative court of Paris to seek the quashing of Mbappe's contract.
The lawyer added: "We are going to proceed in the form of a graduated response. We are going to see little by little what is the capacity of the French professional footballing bodies and also regulatory and administrative authorities to react to our appeals and our requests.
"As time goes by, we will increase the pressure."
LaLiga has also complained to UEFA, European football's governing body, about PSG, as well as Manchester City.
Javier Tebas, the Spanish league's president, this week claimed the agreement between PSG and Mbappe was "an insult to football".
Branco said LaLiga would "go upmarket... harden our game" if necessary, saying it considers the French capital giants to be spending on a "fraudulent" scale.
That accusation has been consistently denied by PSG.