After both the LaLiga and the RFEF had rejected Barca's attempts to register Dani Olmo and Pau Victor, they were overruled temporarily by the High Sports Council this week.
Atletico Madrid, Las Palmas and Real Valladolid have all declared oppostion and have now been joined by Espanyol and Valencia.
Espanyol's statement read: “RCD Espanyol wishes to express its concern and worry about the situation that has arisen in Spanish football following the resolution adopted and made public by the High Council for Sports, in the cases of footballers Dani Olmo and Pau Víctor of FC Barcelona.
"The decision to grant an urgent, provisional precautionary measure until the appeal is finally resolved has created a very dangerous precedent and poses a threat to the integrity of the competition, as other clubs, as well as the RFEF and LaLiga, have denounced in various statements and communications.
"RCD Espanyol, which remains fully committed to the competition, has strictly complied with the regulations on economic control and will continue to do so, despite the tensions, limitations and difficulties that these economic rules generate.
"As has been stated in various forums, we understand that the rigorous economic control applied in recent years has ensured the sustainability of many clubs. The Sports Law itself includes and protects this economic control of LaLiga and, however, with this resolution, the norm approved and accepted by all professional clubs is put at risk.
"The main players in football in this country: LALIGA and the RFEF have maintained a common and firm position from their respective authorities throughout the process. However, the CSD has exposed and discredited both institutions with its decision. RCD Espanyol urges a review of this resolution that generates uncertainty and threatens the principles of equality and justice in the competition.”
Meanwhile, Valencia have also voiced their opposition.
Their statement read: "Valencia CF wishes to express its disagreement with the decision taken by the Higher Sports Council , which is contrary to the position held by both LaLiga and the RFEF . We understand that it is detrimental to the integrity of the competition and also puts at risk the fundamental pillars of the economic management of professional football.
"On the other hand, Valencia CF is concerned about the impact that this measure may have on LaLiga 's Economic Control , a regulation approved by all clubs that has been key to guaranteeing the financial sustainability of Spanish football.
"This decision threatens a system that has been recognised both in the Sports Law and in other countries and is also dangerous for the future of the competition."