Norwegian cross country skier Therese Johaug, a seven-time world champion and former Olympic gold medallist, has failed a drug test, the Norwegian ski federation announced on Thursday.
The 28-year-old tested positive for traces of the banned anabolic steroid clostebol during an out-of-competition control on September 16.
Johaug won relay gold at the 2010 Vancouver Olympics and silver in the 30km freestyle and bronze in the 10km classical in Sochi 2014.
The federation said the skier was unaware that the substance was contained in a lip cream called Trofodermin she used to treat burns during a training session at altitude in Italy late August.
National skiing team doctor Fredrik Bendiksen took responsibility and announced later Thursday his immediate resignation.
"I'm devastated to find myself in this situation which I find extremely complicated and surreal," said Johaug.
"I find it unfair and unjustified even if, of course, I'm aware as an athlete I'm responsible for the substances I use."
No sanction has yet been announced.
Per Wiik Johansen of Antidoping Norge, the country's drugs control body, said Johaug had tested positive for small quantities of the steroid.
"Nothing indicates that that would have performance-enhancing effects," he said.
But Antidoping Norge legal expert Brita Rokenes warned that Johaug could face up to four years' exclusion from competition should she be found guilty of doping.
It is the second doping incident to shake the Norwegian federation recently after cross country skier Martin Sundby was suspended for two months and stripped of his 2015 overall World Cup and Tour de Ski titles for unauthorised use of medication to treat asthma.
The Norwegian federation said it assumed full responsibility for the Sundby case having failed to request the required authorisation.