The World Anti-Doping Agency said Saturday it would investigate allegations that disgraced Chinese swimming star Sun Yang had broken the terms of his ban from the sport.
The Times newspaper reported that the three-time Olympic champion had been training in government-funded facilities despite his four-year suspension.
It said he had been photographed in a "performance sport facility" as he builds up to a potential comeback at the Paris 2024 Olympics.
If Sun is found to have broken the terms of his ban his penalty could be restarted, which would rule him out of the Paris Games, the Times report said.
WADA spokesman James Fitzgerald told AFP that the anti-doping body took the allegations "very seriously".
He said: "We are looking into the matter and, as part of that, we will follow up with the relevant entities, including the international swimming federation (FINA), to gather more information and to be in a position to determine whether the swimmer has breached the terms of his suspension, as per the decision of the Court of Arbitration for Sport (CAS) of 22 June 2021."
CAS banned Sun in June for refusing to give a sample to doping inspectors.
At the end of a long-running and controversial case, the Lausanne-based court reduced its original ban of eight years after Sun, 30, appealed to Switzerland's federal supreme court over alleged bias.
The new ban of four years and three months, backdated to February 2020, ruled the 1,500m freestyle world record-holder out of the Tokyo Olympics and the 2022 Asian Games in his home city of Hangzhou.
Sun, who was banned for three months in 2014 for a separate doping offence, will be eligible to return in time for the Paris 2024 Games, although he will be 32 by then.
The swimmer, who has been dogged by controversy throughout his career, has always maintained his innocence in the murky events of September 2018 when doping inspectors visited his home.
The 11-time world champion said the testers were not qualified or authorised.