Britain's double Olympic champion Mo Farah missed two drug tests in the run-up to the 2012 London Games, the Daily Mail reported on Thursday.
The Mail's story comes at an awkward time for the 32-year-old Farah following recent doping allegations against his coach, Alberto Salazar.
Farah backed out of a meeting in Birmingham earlier this month in the wake of accusations made against Salazar and Galen Rupp, Farah's American training partner. The two men have denied the claims, while Farah has not been accused of doing anything illegal.
However, the Mail said the London Olympics 5,000 and 10,000 metres champion Farah had put his participation at the 2012 Games in jeopardy by missing two tests around the time he started training under Salazar in February 2011. UK Anti-Doping (UKAD) rules state an athlete who misses three tests in any 12-month period (reduced from 18 months since 2013) can face a ban of up to four years.
Britain's Christine Ohuruogu was suspended for 12 months in 2006 after missing three tests and falling foul of the World Anti-Doping Agency's "whereabouts" system, which forces athletes to select an exact location for one hour every day in case they are needed for testing.
According to the Mail, Farah's first missed test appears to have occurred in early 2010, several months before he joined up with Salazar. It was also six months before he broke David Moorcroft's 28-year-old British 5,000m record and became the first Briton to complete the distance in under 13 minutes.
But the second missed test appears to have been scheduled once Farah had started working with Salazar. It should have taken place at Farah's home in Teddington, London, but the athlete appealed to UKAD, claiming that he did not hear his doorbell.
The Mail, citing correspondence it had seen between lawyers, Farah, and Salazar, made clear the Somali-born athlete's concerns about a suspension. The British newspaper added that Salazar had warned Farah on May 11 of that year: "If you miss another test, they will hang you."
Salazar, under whose guidance Farah has won five global titles over 5,000m and 10,000m, is head coach at the prestigious Nike Oregon Project in Portland and an "unpaid consultant" for UK Athletics, which has launched a review into the American's relationship with the governing body and Farah.
Farah has said he intends to stand by his coach unless he is proven to have done wrong.
Wednesday saw the three-time world champion announce he would compete in next week's Diamond League meeting in Monaco, which will be the first time he has run competitively since the allegations against Salazar were aired in a BBC documentary. "Good to be out on the track in France today, getting ready for my next race in Monaco!! Focusing on my training!!!!" tweeted Farah on Wednesday.
A week after the Monaco meet, Farah is due to compete in the Anniversary Games at the Olympic Stadium in London.