Russia's best male gymnast could miss the Olympics after he became the latest to test positive for the banned substance meldonium.
Nikolai Kuksenkov withdrew from the national championships after being informed of the failed doping test on Saturday, a day after winning the individual all-around title.
Kuksenkov, a former European champion, had not taken meldonium since August, Russia team coach Valentina Rodionenko told the Russian Ves Sport agency.
"As yet, no one knows how long a period is needed for meldonium to leave the body," she said.
"It's a kind of wild, wrong situation, which needs to be sorted out with joint effort."
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Kuksenkov competed for Ukraine under the name Mykola Kuksenkov before taking Russian nationality in 2013.
He won European championship gold in the team all-around in 2014, and gold in the same event at last year's inaugural European Games.
Kuksenkov's is the first known doping case in gymnastics involving meldonium.
More than 20 other Russians across all sports have tested positive for meldonium, which was banned on January 1, with at least 123 cases worldwide.
Also, Russian Pavel Kulikov, an Olympic hopeful in skeleton, said in a statement he failed a test for meldonium.
Kulikov finished 12th at the world championships in Austria in February.
