PARIS, Dec 13 (Reuters) - Former French Olympic swimmer Yannick Agnel, who was placed under formal investigation last week for suspected rape of a minor, has admitted to a sexual relationship with a 13-year-old girl, a public prosecutor said on Monday. The 29-year-old Agnel, who won two Olympic golds at the 2012 Games in London, was arrested in Paris on Thursday and transferred to Mulhouse in eastern France for questioning. French prosecutors said the alleged offences occurred around 2016. Prosecutor Edwige Roux-Morizot told a news conference on Monday that prosecutors believe that the age difference between Agnel and the girl is sufficient to show there was coercion despite the former Olympian saying there had been none. Agnel"s lawyer declined to comment. At the time of the alleged offences, sex between an adult and a child younger than 15 was illegal but could only be prosecuted as rape if there was proof the minor was coerced through violence, constraint or surprise. "When there is a ten-year age difference, this is considered by the law to be a case of constraint," Roux-Morizot told Monday"s news conference. Agnel had told investigators he "did not have the feeling that there was constraint", Roux-Morizot said. "I think that he regrets not thinking seriously enough about the age difference at the time," the prosecutor added. In April this year France, adopted legislation that characterises sex with a child under the age of 15 as rape and punishable by up to 20 years in jail, bringing its penal code closer to many other Western nations, but the law cannot be applied retrospectively. The investigation was triggered after the daughter of one of Agnel"s former coaches filed a complaint last summer, citing sexual acts committed around 2016, French media reported. Agnel, who won the 200 metre individual freestyle and 4x100 metre freestyle relay in London, now works as a sports and media consultant.