The European Commission said its antitrust inspectors raided the premises of two French grocery retailers earlier this week as part of a preliminary investigation. It did not identify the companies and said the move did not mean they were guilty.
Casino said separately that Commission agents visited its Paris headquarters this week, but did not provide further details.
Le Figaro newspaper reported Intermarche's headquarters were also raided. The company told Reuters it "confirms the European Commission's ongoing probe," but declined to give details.
"The Commission has concerns that two grocery retail companies may have violated EU antitrust rules that prohibit cartels and restrictive businesses practices," it said in a statement.
"The fact that the Commission carries out such inspections does not mean that the companies are guilty of anti-competitive behaviour nor does it prejudge the outcome of the investigation itself," it added.
There is no legal deadline to complete inquiries into anti-competitive conduct. Their duration depends on a number of factors, including the complexity of each case and the extent to which the companies concerned cooperate with the Commission.
Casino and Intermarche announced in 2014 a purchasing alliance in France that was dissolved last year when Casino and France's Auchan sealed a global buying deal.
France has seen a flurry of purchasing alliances in recent years as supermarkets battle to keep prices down to counter fierce competition from rivals offering prices at discounts and the looming shadow of U.S. internet giant Amazon.
(Reporting by Jan Strupczewski, Domininique Vidalon; Editing by Louise Heavens and Mark Potter)
By Jan Strupczewski and Dominique Vidalon