A dark cloud has been cast over the looming first El Clasico this season in Spain.

L'Esportiu says Spanish police are investigating a case of pressure being put on an assistant referee to benefit Real Madrid in the upcoming Clasico against Barcelona.

It's alleged the match official was expected to form part of the refereeing team for the game on November 21 at the Santiago Bernabeu.

The report suggests another referee told him that he had been given instructions to benefit Los Blancos in the game.

It is also claimed an Anti-Corruption Prosecutor from Barcelona has had knowledge of the case since October 19 when Jacinto Vicente, a lawyer who specialises in sport, submitted a document detailing the complaints raised by the referee who had been pressured.

Vicente requested that the name and personal data of the official did not appear in the document because it could lead to concerns of prejudice against him -- both personally and professionally.

Spanish police are now responsible for investigating the case by contacting the persons related to the events and clarifying what they can.