Reigning Olympic and London marathon champion Jemima Sumgong is the latest Kenyan athlete to fail a drugs test.
The 32-year-old tested positive for banned substance EPO in an out of competition test carried out by athletics' governing body the IAAF.
Sumgong - the first Kenyan to win Olympic marathon gold - was due to defend her London title on 23 April.
Kenya was last year declared in breach of anti-doping rules, and athletes underwent special testing for Rio 2016.
The East African country was deemed "non-compliant" by the World Anti-Doping Agency, but was reinstated before last summer's Games.
Between 2011 and 2016, more than 40 Kenyan track-and-field athletes failed doping tests.
Among those sanctioned was female marathon runner Rita Jeptoo, 36, who was banned for four years following a positive test for performance-enhancing drug EPO in 2014.
Sumgong is provisionally suspended, and she will face sanctions if her B-sample also tests positive.
Eunice Kirwa of Bahrain took silver behind Sumgong in Rio, with Ethiopia's world champion Mare Dibaba claiming bronze and another Ethiopian, Tirfi Tsegaye, fourth.
"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."
London Marathon organisers said they were "extremely disappointed" by Sumgong's positive test, adding: "We are determined to make marathon running a safe haven from doping."
In 2015, the Sunday Times claimed the London Marathon had been won seven times in 12 years by athletes who had recorded suspicious blood scores.
That followed details of 12,000 blood test results from 5,000 athletes published by the newspaper, in partnership with German broadcaster ARD - BBC
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