Olympic 100 metres breaststroke gold medallist Rūta Meilutytė is facing a suspension of up to two years and could be ruled out of Tokyo 2020 after she missed three drugs tests in 12 months.
The Lithuanian Swimming Federation (LSF) confirmed it had received notification from the International Swimming Federation (FINA) regarding Meilutytė's failure to appear for the three doping controls.
The LSF said Meilutytė, who produced a stunning upset to win gold in the 100m breaststroke at London 2012 and is the short course world record holder, had accepted responsibility.
According to the national governing body, drugs testers had been unable to contact the 22-year-old at the address she had provided on the whereabouts system on three separate occasions – April 22 and August 19, 2018 and on March 28 this year.
Meilutytė, an outspoken critic of athletes who cheat, claimed a failure to correctly fill out her whereabouts information had been the reason behind the missed tests.
FINA has given her 12 days to provide clarification.
Under the World Anti-Doping Code, athletes who miss three tests in 12 months can be banned for up to two years, although the length of the sanction can be reduced depending on the degree of the competitor's fault.
Kristina Jagminiene, director of the Anti-Doping Agency of the Republic of Lithuania, said it was a "pity" that Meilutytė – a household name in Lithuania following her London 2012 triumph – could be banned because of "such a shameful mistake".
LSF President Emilis Vaitkaitis claimed it was the "worst news" the federation could have received.
"It is very disappointing that such an anti-doping rule violation has been recorded by an athlete who has been reporting this data for 15 years," Vaitkaitis said.
"An athlete who has been tested an unimaginable number of times and an athlete who always spoke out loud for clean sport.
"It is twice as disappointing that the penalty can be imposed at the start of the Olympic selection, when all the preparedness plans are finally aligned."