Kenya's Olympic marathon gold medallist Jemima Sumgong has failed an out-of-competition drugs test, the International Association of Athletics Federations (IAAF) have confirmed.
Sumgong tested positive for the banned substance erythropoietin (EPO), a hormone which boosts red blood cells.
The IAAF have revealed that a case has been opened this week against the runner, who triumphed in the women’s marathon at Rio 2016 in a time of 2 hours 24min 04sec.
She became the first Kenyan woman to claim the title, finishing in front of Bahrain's Eunice Kirwa and Ethiopia's Mare Dibaba.
"We can confirm that an anti-doping rule violation case concerning Jemima Sumgong (Kenya) has commenced this week," the IAAF said in a statement.
"The athlete tested positive for EPO (Erythropoietin) following a no-notice test conducted in Kenya.
"This was part of an enhanced IAAF out-of-competition testing programme dedicated to elite marathon runners which is supported by the Abbott World Marathon Majors group."
The Kenyan faces the prospect of being handed a ban by the governing body, should her B-sample come back positive.
Sumgong was poised to be on the start-line for the London Marathon on April 23.
The Kenyan won last year's race in 2:22:58.
London Marathon chief executive Nick Bitel has confirmed the positive followed an Abbott World Marathon Majors (Abbott WMM) funded test and vowed to take strong action.
"She [Sumgoing] is currently the leader in the Abbott WMM Series X standings which concludes in Boston on 17 April," he said.
"Under Abbott WMM rules, if she were to finish top of the rankings at conclusion of the Series, an athlete who fails a drug test and is banned is not eligible to win the Series or to receive any prize money.
"We are determined to make marathon running a safe haven from doping.
"In addition to testing operated by the IAAF and national federations, the Abbott WMM has set up one of the largest private testing pools of athletes in sport, with the aim being to test 150 competitors out of competition a minimum of six times a year.
“Sumgong's provisional positive test came from one of the Abbott WMM-funded out of competition tests.
"London Marathon Events Ltd and AbbottWMM will continue to do everything we can to ensure cheats are caught and do not benefit from cheating.
"The London Marathon has always been at the forefront of the fight against doping, 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.
"In addition, London Marathon Events will take legal proceedings against athletes who have been disqualified from its events for failed tests if they do not repay money that they have received from the event."
Tim Hadzima, Abbott WMM general manager, has stated that they will not name a women’s champion at the end of the series, while the case is ongoing against Sumgong.
He added that if true, the case proved "we are gaining ground in our long-standing fight against doping".
"Sumgong currently leads the standings for AbbottWMM Series X, which concludes at the April 17 Boston Marathon. Under AbbottWMM rules, any athlete who has been found guilty of any anti-doping rules enforced by the IAAF, WADA, National Federations or any of the individual AbbottWMM races is not eligible to win a AbbottWMM Series title," Hadzima said.
"Given our policies, until the conclusion of Jemima Sumgong’s case and any potential appeal process, we will not name a Series X women's champion.
"In Boston, we will celebrate the hundreds of thousands of hard-working athletes who have crossed the finish lines of our six races in the past year.
"As we honour their accomplishments, we also will award titles to the Series X winners of our inaugural wheelchair championship as well as name our men's Series X champion."
Kenya is an athletics powerhouse, particularly in long and middle-distance running, and topped the medals table at the 2015 IAAF World Championships in Beijing.
The country, however, has regularly been tainted with drugs problems.
Around 40 athletes from the country have tested positive for banned drugs since 2012, including three-times Boston Marathon winner Rita Jeptoo, who failed for EPO in 2014.
A new law criminalising doping has been introduced in the country, which was declared non-compliant by the World Anti-Doping Agency in May to put participation at Rio 2016 in jeopardy.
WADA said Kenya were in breach of anti-doping rules before the situation was resolved in time for the Olympics.