Chelsea winger Mykhailo Mudryk failed an anti-doping test, the Premier League club confirmed.
The Blues say they were contacted by the Football Association following an “unfavorable result from 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 November 28 and has missed the last five games in all competitions.
He was included in the matchday squad for the Blues’ victory over Aston Villa on 1 December, but did not feature.
Head coach Enzo Maresca said publicly that Mudryk’s absence was due to illness.
