PARIS — France’s top financial prosecutor opened a preliminary investigation on Monday into whether President Emmanuel Macron’s chief of staff had breached conflict-of-interest rules in previous positions, the first accusation of corruption against the president’s inner circle since he took office a year ago.

The chief of staff, Alexis Kohler, 45, has been described by the newspaper Le Monde as a member of the “trio that rules France,” alongside Mr. Macron and his special adviser, Ismaël Emelien. The inquiry follows a complaint filed by the anticorruption group Anticor, which accused Mr. Kohler of influence peddling and of breaking conflict-of-interest rules.

The prosecutor’s office said it had opened an inquiry after news reports detailed the “conditions and circumstances” under which Mr. Kohler dealt with cases involving an Italian-Swiss shipping and cruise firm, the Mediterranean Shipping Company, while he was at the agency that handles state holdings and at the Economy Ministry. He went on to join the Mediterranean Shipping Company, or M.S.C., as financial director in 2016.

The complaint stemmed from findings of the investigative website Mediapart, which revealed last month that Mr. Kohler was related to the owners of M.S.C.: His mother is a cousin of Rafaela Aponte, who co-founded the company with her husband, Gianluigi, in 1970.