Kenya's Jemima Sumgong will be barred from defending her London Marathon title later this month after it emerged she had failed a drugs test in February.
Sumgong, who also won the Olympic marathon title in Rio last summer, has been suspended pending the test on her B sample after an initial test was positive for the banned blood-boosting drug EPO.
A statement from Nick Bitel, the chief executive of London Marathon Events Limited said: "We are extremely disappointed to learn that Jemima Sumgong (Kenya) failed an out of competition drugs test in February. She is currently suspended from competition pending the B test and the outcome of the investigation.
"Sumgong will therefore not run in London on April 23 to defend the title she won last year.
"She is currently the leader in the Abbott World Marathon Majors (Abbott WMM) Series X standings which concludes in Boston on April 17. 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."
The sport's global governing body the IAAF confirmed Sumgong's positive test.
"The IAAF can confirm that an anti-doping rule violation case concerning the athlete Jemima Jelagat Sumgong (Kenya) has commenced this week," an IAAF statement said.
"The athlete tested positive for EPO following a no-notice test conducted by the IAAF 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."