Chelsea forward Mykhailo Mudryk has been provisionally suspended by the Football Association after failing a drugs test.
The positive test was for the prohibited substance meldonium and occurred when Mudryk was out of the country and not with Chelsea.
Ukraine played in Albania and Georgia last month.
Mudryk, who faces a potential ban of up to four years under FA rules, said in a statement this is "a complete shock" and he has "not done anything wrong".
He is not training at Cobham currently and is on an individual fitness programme at home, and is thought to be devastated by the news.
The player believes his sample was contaminated, with nothing thought to have changed in his diet since he passed a UK Anti-Doping test in August.
Chelsea have launched their own investigation and said the 23-year-old insists he "has never knowingly used any banned substances".
The club are prepared to give him all the help and support he may need, including access to mental health professionals if required.
Mudryk has not been in the Chelsea squad for the last four games, with head coach Enzo Maresca putting his absence down to illness.
The Ukraine winger, signed from Shakhtar Donetsk for £88.5m in January 2023, last played for Chelsea in a 2-0 win at German side Heidenhem in the Conference League on November 28.
Mudryk, with three goals in 15 games this season, most recently appeared on the bench for Chelsea as an unused substitute on December 1 in a 3-0 win over Aston Villa at Stamford Bridge.
FA rules dictate a player is informed of a positive test and provisionally suspended before a decision is taken on a formal charge. Mudryk has the opportunity during this period to provide an explanation for the result.
Chelsea's statement read: "Chelsea Football Club can confirm the Football Association recently contacted our player Mykhailo Mudryk concerning an adverse finding in a routine urine test.
"Both the club and Mykhailo fully support The FA's testing programme and all our players, including Mykhailo, are regularly tested.
"Mykhailo has confirmed categorically that he has never knowingly used any banned substances. Both Mykhailo and the club will now work with the relevant authorities to establish what has caused the adverse finding.
"The club will not be commenting any further."
Mudryk, in his own statement, added: "This has come as a complete shock as I have never knowingly used any banned substances or broken any rules, and am working closely with my team to investigate how this could have happened.
"I know that I have not done anything wrong and remain hopeful that I will be back on the pitch soon. I cannot say any more now due to the confidentiality of the process, but I will as soon as I can."
Anti-doping breaches are 'strict liability offences'
Sky Sports News chief reporter Kaveh Solhekol:
"What's important in this case is the 'A' sample has tested positive. We are still waiting the result of the 'B' sample, so I think that is very important for Mykhailo Mudryk.
"He has missed the last five games for Chelsea. Enzo Maresca, when he has been asked about Mykhailo Mudryk's absence, has said that the player is ill. We know the player was genuinely ill with a virus.
"In the meantime, Chelsea have launched their own investigation - and it should be noted that the player did pass a routine anti-doping test in August.
"It's important to stress as well that anti-doping breaches are strict liability offences. That means you as a player are responsible for what is in your body. It does not matter how it got there - it is a strict liability offence.
"Mykhailo Mudryk himself has been on social media. He said that he is in a state of complete shock. He said he never knowingly used a banned substance, and he says he's done nothing wrong and hopes to be back on the pitch on soon.
"The player is confident he will be cleared because he feels has done nothing wrong and thinks there is strong evidence which supports his position."
