Spanish football faces a potentially huge match-fixing scandal after an unnamed linesman alleged he had been told to favour Real Madrid in next month’s clásico against Barcelona.

The linesman said he did not want to be identified because of concerns over “possible reprisals” but he made a formal complaint through a lawyer to the anti-corruption investigating magistrate in Barcelona, in which he claimed to have been told to favour Madrid – first by another official and then, over the phone, by a member of the Spanish Football Federation’s referees’ committee.

The linesman alleged he was told officials had already acted to influence the outcome of games and that another linesman has been subjected to similar pressures. According to his lawyer, Jacinto Vicente Hernández, an expert in sports law based in Barcelona, the linesman wanted to go public with the allegations but was persuaded to make a formal, legal statement to the anti-corruption investigating magistrate instead. The Guardia Civil has also been informed.

The story was broken by the Catalan newspaper L’Esportiu on Wednesday and followed up by Cadena Cope radio station. It has had a huge impact in Spain, even if there are doubts anything will come of it. The denuncia or formal complaint made by the linesman through his lawyer, dated 19 October, has been published. In it, the linesman recognises a telephone conversation he says took place was not recorded and that there is no concrete proof of it, but he hopes the other linesman will also come forward.

José Ángel Jiménez Muñoz, who is reported as being the referees’ committee member accused of having made the telephone call to the linesman, said on Wednesday night he had not yet had the chance to read the complaint but said of 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, she 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 a dangerous story in terms of the prestige of La Liga.” The claims have not been substantiated.

Miguel Cardenal, Spain’s 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 chosen for el clásico, alleges that an official spoke to him in September and suggested that, should he be given the chance to officiate, he should seek to influence the game in Madrid’s favour. The linesman was allegedly told the order had come from “someone inside [the refereeing authorities]” and that it was better for the linesman to try to fix the game than a referee as the lead official is subject to greater media scrutiny.

He claims to have been told this had already happened this season and that, as it caused a stir, it was better to be more “subtle”. The linesman says he was not comfortable with the suggestion and that the official replied nor was he but as they are constantly rated by the referees’ committee, and their careers depend upon that committee, it would be wise to follow orders.

The linesman alleges that a few days later he received the phone call from Jiménez Muñoz, who had been made aware of the linesman’s reluctance to participate. His lawyer suggested he call back and record the conversation but the linesman said the alleged call had come through a switchboard so he could not.

The La Liga president, Javier Tebas, said: “This allegation seems a bit far-fetched. It makes no sense, when the referee for this game has not been designated [yet]. The league is not set up for anybody, not from the league, or the Federation or anywhere.”

According to the linesman’s lawyer, he wanted to speak out in order to relieve the pressure he felt he was being put under. Having made the formal statement, the next step will be for an investigation to be opened, should it be considered there are sufficient grounds to do so. But the linesman admits there is no hard evidence. Real Madrid have yet to comment.