Braithwaite, 28, joined Barcelona on 20 February to cover while Ousmane Dembele is out injured for six months.

La Liga allows clubs to replace a player who is out for over five months, but Leganes cannot replace Braithwaite.

"These are unfair regulations," a Leganes statement said.

"It goes against the integrity of the competition and the right of all clubs to compete in equal conditions and has caused enormous and serious damage to the club."

Barcelona signed the Danish striker for 18m euros (£15m) in a contract that will run until the end of June 2024.

Denmark striker Braithwaite signed a four-year-deal at Leganes - who are in the La Liga relegation zone - in July following a spell on loan from Championship side Middlesbrough last season.

He scored eight goals in 27 games since joining them permanently and has six of Leganes' 18 league goals so far this season.

The Spanish Football Federation (RFEF) rejected Leganes' request on the grounds that there is no Fifa, RFEF or La Liga ruling that allows a club to sign a player outside the transfer window to replace one who has left to cover for a long-term injury absence.

But the federation is going to begin the process of proposing a change to the rule.

"The RFEF states that allowing Leganes' request could lead to a chain of transfers outside the window which could pose a big threat to the integrity of the competition and equality of participants," the Leganes statement added.