Barcelona coach Luis Enrique has called for an investigation into claims an assistant referee was told to favour Real Madrid in November's El Clasico.
The unnamed official made the claims in a document presented to anti-corruption authorities in Spain.
The Catalan side currently lie third in La Liga three points behind leaders Real, who beat Celta Vigo on Saturday.
Barca boss Enrique said: "It isn't nice, so I hope it's fully investigated to avoid any risk to our football."
The official has not been identified.
As his side prepare to host Eibar in the league on Sunday, Enrique, 45, said that he did not want to speculate on the claims.
He added: "Referees already have a difficult enough job without adding more fuel to the fire.
"What seems logical and licit to me is that it is fully investigated to see if it is true or it is something that someone has invented."
Real have refused to comment.
Spanish police have also been informed of the allegation.