Linesman claims he was told that this was not the first time an official had been approached this season.

A linesman who could be appointed to officiate at next month’s clásico, has claimed he was asked to favour Real Madrid over Barcelona.

The linesman has requested anonymity because of fears over “possible reprisals” but he made a formal complaint through a lawyer to the anti-corruption investigating magistrate in Barcelona. He claims to have been told to favour Madrid – initially by another official and then by a member of the Spanish Football Federation’s referees’ committee.

The linesman alleged that he was told that the case was not unique and that other officials had already acted to influence the outcome of game. According to his lawyer, Jacinto Vicente Hernández, the linesman wanted to go public with the allegations but he persuaded his client to make a formal, legal statement to the anti-corruption investigating magistrate instead.

The formal complaint made by the linesman says that a telephone conversation took place but acknowledges it was not recorded and that there is no tangible evidence,. But he expresses the hope that in the light of his admission, the other linesman will also come forward.

José Ángel Jiménez Muñoz has been reported as being the member of the referee’s committee accused of having made the telephone call to the linesman, but he dismissed the allegations.

“I have no idea where they come from. It is like something out of a Kafka novel.”

Barcelona’s vice-president Susana Monje said: “I hope it does not prove to be true.”

Later, he added: “We will possibly discuss at it a meeting of the board. All I can say at the moment is that if it is confirmed, it will be dangerous story in terms of the prestige of La Liga.”

Miguel Cardenal, the Spanish secretary of state for sport, said: “This is an extremely serious accusation and is extraordinarily alarming. This needs to be resolved as soon as possible. We cannot be talking about threats without knowing who is it. No one should be scared. It cannot be reasonable that in 2015 someone can fear reprisals for making an accusation like this.”

The linesman, who forms part of one of the officiating teams that could be designated for the clásico, alleges that an official spoke to him in September and suggested that, should they be given the chance to officiate the clasico, he should seek to influence the game in Madrid’s favour.

A few days later the linesman said he received a phone call from Jiménez Muñoz, who allegedly administered additional pressure on the official to agree to favour Madrid.

Real Madrid have yet to comment on the allegation.