The 32-year-old is also the reigning London Marathon champion.
Jemima Sumgong, the first Kenyan woman to win Olympic marathon gold when she triumphed at Rio in 2016, has failed an out-of-competition dope test, reports claimed on Friday.
The 32-year-old, who is also the reigning London Marathon champion, tested positive for the banned blood booster EPO in a test by the International Association of Athletics Federations (IAAF) in her native Kenya, the BBC reported.
"We can confirm that an anti-doping rule violation case concerning Jemima Sumgong has commenced this week," www.bbc.com quoted the IAAF as saying in a statement.
"The athlete tested positive for EPO following a no-notice test in Kenya."
The IAAF did not respond when asked by AFP for confirmation of the report early Friday.
Sumgong starred at the 2016 London Marathon, defying the odds to win despite suffering a bruising fall.
Steeled by her success in London, she then became the first Kenyan woman to win Olympic marathon gold in Rio.
Sumgong defeated Ethiopia's world champion Mare Dibaba to confirm her status as the world's number one marathon runner of the year.
Before claims of a positive drugs test emerged, Sumgong said she was looking forward to returning to London to defend her title on April 23.
"London is the marathon every runner wants to win," she said. "I can't wait to return to defend my title."
Tim Hadzima, general manager at Abbott World Marathon Majors, organiser of the world's largest marathons including London, said the organisation was "distressed" by the reports, but said that "if true, they indicate that we are gaining ground in our long-standing fight against doping".
At the Rio Olympics, Sumgong defied temperatures of 28 degrees Celsius (82F) to claim an historic gold medal in a race which finished at the city's famed Sambodromo.
"It was very hot but everybody had to get through the heat. I had to control my body and listen to my body very carefully," said Sumgong, who added that victory made up for her disappointing showing at the 2008 Beijing Olympics.
"I was in Beijing but I was pretty disappointed that I wasn't able to win a medal or make it on the podium, but I knew one time, one day, I'd be somewhere," she said.
"I was never worried that I'd lose this. At the 40km I knew the gold was mine."
Earlier this year, Sumgong was one of a number of top Kenyan athletes who welcomed a new initiative to stop doping, which has tarnished their image, in which they agreed to be monitored by doctors appointed by the IAAF and Athletics Kenya.
"It will be easy for us now to communicate with these doctors before we take any medicine when the need arises," said Sumgong.
In July last year, an investigation by German television channel ARD and Britain's Sunday Times newspaper alleged that doping is rife at the elite training centre in Iten.
Sumgong's former training partner, the 2014 Chicago and Boston Marathon champion Rita Jeptoo, is serving a four-year ban after also testing positive for EPO.
Athletics Kenya chief Jackson Tuwei warned that any athlete who failed to comply would not be selected to represent Kenya in international competitions.
"Forty-nine athletes have been found to have violated the WADA (World Anti-Doping Agency) code in the past five years but were cautioned according to the laws of the land and WADA code," said Tuwei.
News of Sumgong's test was welcomed by other athletes. US distance runner Emma Coburn, a bronze medallist in the 3,000m steeplechase at last year's Olympics, applauded the IAAF's out-of-competition testing.
"Out of competition testing is so important!! Well done, IAAF. I hope to see more productive results from no-notice out of competition tests," Coburn wrote on Twitter.