Chelsea have confirmed they have contacted the Football Association after winger Mykhailo Mudryk came back with negative results in a routine urine test.
The Ukraine international, who has made 15 appearances for Chelsea this season, has denied taking any banned drugs.
A Chelsea statement read: “Chelsea Football Club can confirm that the football club has recently contacted our player Mykhailo Mudryk regarding an adverse finding in a routine urinalysis.
“Both the club and Mykhailo fully support the FA's testing program and all our players, including Mykhailo, are regularly tested.
“Mykhailo categorically confirmed that he was never aware of the banned substances.
“Both Mykhailo and the club will now co-operate with the relevant authorities to investigate the serious findings.
“The club will have no further comment.”
Citing multiple sources, The Athletic reported that Mudryk tested positive for Meldonium after playing for Ukraine in November. Meldonium can help an athlete recover from strenuous exercise and was added to the World Anti-Doping Authority (WADA) banned list in January 2016.
Mudryk said on social media: “I can confirm that the sample I gave the FA has been informed that it contains banned substances.
“It is a complete shock as I have never known or broken the rules and never used banned chemicals and I am working closely with my team to investigate how this could have happened.
“I know I didn't do anything wrong and I hope to be back on the pitch soon. Because of the secrecy of the process, I can't say now, but I will do it as soon as possible.”
Mudryk, who has missed the last five games against Chelsea citing illness, joined the west London club from Shakhtar Donetsk last January in a move that could rise to £88m. He has struggled for a regular place in the first team and has mainly played for second-tier clubs that have competed in cup competitions this season.