Mykhailo Mudryk supplied an "adverse finding in a routine urine test" in the FA's anti-doping programme, Chelsea confirmed on Tuesday.
A club statement issued by the club said Mudryk "confirmed categorically that he has never knowingly used any banned substances" and the club will "work with the relevant authorities to establish what has caused the adverse finding."
Mudryk joined Chelsea in January 2023 in a transfer from Ukrainian side Shakhtar Donetsk worth €100 million.