French Rugby Federation (FFR) president Bernard Laporte was questioned on Tuesday over his links with billionaire Montpellier owner Mohed Altrad, sources in the investigation said.
Altrad and the chief organiser of the 2023 Rugby World Cup being held in France, Claude Atcher, were also questioned, along with two other people.
Laporte, a former sports minister, is being investigated as part of a preliminary corruption probe opened in 2017 by the French Financial Prosecution Office (PNF) following a report by sports ministry inspectors.
Laporte, who is also World Rugby vice-chairperson, is suspected of pressuring the appeals committee of the French league to reduce a disciplinary punishment against Montpellier.
Laporte is standing for re-election as FFR chief with the result expected on 3 October.
He is accused of favouritism after it emerged a company he owns had signed an image rights contract with the Altrad Group, who now sponsor the France national team, in early 2017.
He eventually pulled out of the €150 000 deal but the case was investigated by the French sports ministry before being passed to the public prosecutor.
There are also claims a fine for Altrad's side was reduced from €70 000 to €20 000 after Laporte intervened.
Laporte has coached France, Stade Francais and Toulon and led the southern club to three straight European Cups.