Reigning Olympic and London marathon champion Jemima Sumgong of Kenya has tested positive for the blood booster erythropoietin (EPO) in an out-of-competition test in her home country, the IAAF confirmed, tarnishing the first-ever women's marathon gold medal for the distance running powerhouse.

The IAAF, track and field's international governing body, said Sumgong "tested positive for EPO following a no-notice test conducted by the IAAF in Kenya" and that an anti-doping case had been opened against her.

Reuters reported Thursday that Sumgong's "A" sample has not yet been confirmed by the backup sample, but such confirmations are nearly always routine.

Sumgong, 32, also won the 2016 London Marathon despite falling heavily late in the race. The IAAF statement said she was caught as the result of enhanced testing funded by the World Marathon Majors -- a consortium that includes the event's six most prestigious races.

In a statement posted on the WMM website, general manager Tim Hadzima said: "The Abbott World Marathon Majors is committed to eradicating doping and we will continue to lead the way in introducing and campaigning for aggressive measures. To that end, we recently, in conjunction with the IAAF, built and funded one of the largest targeted testing pool of athletes, with an aim of requiring more than 150 individuals to submit to out-of-competition testing a minimum of six times a year.

"As we understand it from reports, one of those out-of-competition tests resulted in Jemima Sumgong's provisional positive results. This demonstrates that, while there is more work to do, our efforts have served an important purpose.''

Hadzima's statement reiterated that under a policy rewritten in 2015, Sumgong's potential $500,000 bonus for winning the WMM's yearlong "Series X," which ends with this month's Boston Marathon, would be withheld until the end of her appeals process. Sumgong currently leads the standings. The policy also entitles the WMM to retroactively strip Sumgong of her London appearance fee and prize money.

London Marathon CEO Nick Bitel said in a statement the Sumgong will not race in this month's London Marathon due to the positive test.

"The London Marathon has always been at the forefront of the fight against doping," Bitel said. "The race has a zero tolerance policy towards doping and athletes who are banned for a doping offence are banned for life from the event and any other race organised by London Marathon Events Ltd."

Sumgong's win on a steamy Sunday in Rio de Janeiro last August was cloaked in doubt generated by Kenya's sketchy drug testing history and her own associations. Her former training partner, Rita Jeptoo, who won the Boston Marathon three times and the Chicago Marathon twice, is currently serving a four-year ban for EPO use.

Jemima Sumgong became the first Kenyan to win the women's marathon. Fabrice Coffrini/Getty Images

Jeptoo and Sumgong shared an Italian agent, Federico Rosa, who was charged with administering performance-enhancing drugs at the time of Sumgong's Olympic race. Released on bail, Rosa was at the Rio finish line to greet his client, and the charges were dropped last November.

Sumgong herself tested positive for the banned steroid prednisolone, which is used to reduce inflammation, after she finished second in the 2012 Boston Marathon in a Kenyan podium sweep. Athletics Kenya, the national track and field federation, initially banned her for two years. The IAAF later reversed that decision, ruling that she had received a legal injection for an injury and properly declared it on her doping control form.

After her historic win in Rio, Sumgong told reporters, "In Kenya we are clean, very clean, I assure you we are clean.''

The top U.S. finisher in Rio last August was Shalane Flanagan, who came in sixth and said afterward, "That's all I have. That's what I am.''

Flanagan, who finished seventh in the 2014 Boston Marathon, has already seen two women who beat her in that race convicted of doping violations. Sumgong could be the third.

On Thursday, after news of Sumgong's positive test broke, Flanagan tweeted, "What is real anymore?... "