Chelsea winger Mykhailo Mudryk has failed a drugs test, the Premier League club has confirmed.
The Blues say they were contacted by the Football Association after an "adverse finding in a routine urine test".
Ukraine international Mudryk denies knowingly using any banned substance and Chelsea say they will work to establish what caused the test result.
BBC Sport has contacted the Football Association for further comment.
The 23-year-old last played for Chelsea on 28 November and has missed the past five matches in all competitions.
He was included in the matchday squad for the Blues' win against Aston Villa on 1 December but did not feature.
Head coach Enzo Maresca had said publicly that Mudryk's absence was down to illness.