Alvarez said he had tested positive after Bilbao’s 3-0 home defeat in the semi-finals of Uefa’s second-tier club competition in May.

The Spanish side also lost the return leg 4-1.

The 30-year-old centreback was diagnosed with testicular cancer in 2016 and said he had been taking the medicine as part of his treatment for alopecia.

“After studying the case, we have established that I tested positive because I unintentionally took a hair loss prevention medicine containing a banned substance,” Alvarez said in a social media post yesterday.

“The disciplinary procedure is currently under investigation and subject to confidentiality, so I am provisionally suspended and am not authorised to make any further public statements.

“I would like to say to all of you that I very much regret this situation but, with the support of the club, I am working on my defence in the belief that I will be able to return to the field of play as soon as possible.”

Uefa said they would not comment on the case until a final disciplinary decision had been made.

Bilbao threw their full support behind Alvarez, who came through the club’s youth academy and has made 257 appearances in all competitions for the senior team.