French authorities are investigating whether a Champions League game involving Paris Saint-Germain and Red Star Belgrade was fixed.

Both clubs have strongly denied wrongdoing after L’Équipe reported that a Red Star official was suspected of betting €5m (£4.4m) on the team losing by five goals in a match PSG won 6-1. It is understood, Uefa passed on suspicions to the French authorities after the 3 October game at the Parc des Princes in Group C, which also includes Liverpool and Napoli.

France’s financial prosecutor’s office confirmed an investigation had been opened but declined to comment on the report.

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Red Star, who drew their opening group game 0-0 at home to Napoli, said: “It is with the greatest anger and resentment that Red Star deny today’s report in the French newspaper L’Équipe in connection to doubts about the outcome of the Paris Saint-Germain-Red Star match and the involvement of anyone from Red Star in possible improper actions.

“The allegations from this report can cause great damage to the reputation of our club, and therefore Red Star insists both Uefa and the competent investigating authorities in Serbia and France investigate these doubts to the end and come to the truth.

“Today’s technologies and various other mechanisms have progressed so much that it is simply not possible for this case to be left unresolved. Red Star expects that the truth about this must come in as short a time as possible and that any doubt about the involvement of any member of our club in any possible disruptive actions must be removed.”

PSG expressed “amazement and indignation” and added: “Paris Saint-Germain reiterates its commitment to a fundamental principle of sport, that of the integrity of competitions and therefore refutes any form of practice that could call into question that integrity.”

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Their statement read: “It was with the greatest stupefaction that Paris Saint-Germain discovered this article on the L’Équipe website regarding suspicions of match-fixing concerning the Champions League game with Red Star Belgrade. The club categorically rejects any and all direct and indirect implication in relation to these suspicions and would like to point out at no moment has Paris Saint-Germain been approached by the National Financial Court in charge of the investigation.

“PSG remains at the disposal of the investigators and will not tolerate the slightest attack on its reputation nor that of its officials. To that end the club reserves the right to take legal action against any party that makes defamatory statements against the club and/or its officials.”

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ARJEL, the body in charge of regulating online gambling in France, told L’Équipe it did not notice any suspicious bets on the game.

A Uefa spokesperson said: “Uefa never comments on potential, or the existence of, investigations into alleged match-fixing so as to not compromise the process of such investigations.

“Uefa only communicates on specific cases/matches once disciplinary proceedings have been opened and/or decisions have been taken. Decisions taken by Uefa’s disciplinary bodies related to match-fixing are listed here”