LaLiga clubs last week voted in favour of a €2 billion investment deal with CVC despite ongoing opposition from Madrid, Barca and Athletic.
"Project Impulse," as it's dubbed by the league, will see clubs receive funds to spend on infrastructure and modernisation -- as well as increased spending limits for transfers -- in exchange for handing 11% of LaLiga's revenue from TV rights to CVC for the next 50 years.
However, Madrid, Barca and Athletic, which also have the backing of the Spanish Football Federation, say the project will prove hugely damaging to the game in the country.
"Athletic Bilbao, Barcelona and Real Madrid report that they have taken legal action in response to the agreements adopted by the LaLiga Assembly in relation to the so-called Project Impulse," the three clubs said Wednesday in a joint statement.
"[The agreement] with the CVC venture capital fund is an illegal transaction that causes irreparable damage to the Spanish football sector as a whole, and flagrantly violates the most elementary principles of Spanish sports law and the LaLiga statutes."
The league didn't immediately respond to a request for comment on the suit.
The deal was approved Friday, with 37 of Spain's 42 first- and second-division clubs voting in favour. Sources told ESPN that UD Ibiza had joined Madrid, Barca and Athletic in voting against it, with an unnamed club abstaining.
The opposition of Madrid, Barca and Athletic saw them excluded from the terms of the initiative. Therefore, while the other clubs are due to receive the first cash influx of €400 million in the coming weeks, they will not, but they will retain all of their TV rights income.
Barca president Joan Laporta spoke out against the deal last summer, saying that he would, in effect, be mortgaging off the club's future by signing up for it, even though refusing to do so led to the departure of Lionel Messi.
In a war of words with LaLiga president Javier Tebas, the three teams contend that the CVC agreement has been pushed on clubs suffering financially because of the coronavirus crisis.
Madrid, Barca and Athletic even wrote to fellow clubs earlier this month to offer an alternative proposal called "Project Sustainable" in collaboration with European Super League backers JP Morgan, Bank of America and HSBC.
They said that arrangement, also worth €2bn but over 25 years instead of 50 with an interest rate of 2.5-3%, was a "long-term, sustainable, rational and legal" alternative that would be 15 times more affordable if CVC's growth predictions are met.
Tebas dismissed the offer as "an attempt to break the consensus and generate uncertainty."