The Office of Fair Trading says it is investigating whether DB Apparel UK entered into anti-competitive agreements with retailers John Lewis, Debenhams and House of Fraser between 2008 and 2011 over the Shock Absorber bra.
The regulator said its provisional view was that the manufacturer and the stores agreed on retail prices, in a "serious infringement of competition law."
The office's senior director of services, Ann Pope, said the agency would hear from the companies before making a final ruling.
The companies dispute the claims.
The Shock Absorber was once advertised by tennis player Anna Kournikova under the slogan: "Only the balls should bounce."