Seven London Marathon winners in 12 years have recorded blood level results which suggested they may have been doping, the Sunday Times newspaper reported in the latest set of allegations to have rocked athletics.
It reported that 32 winners of major city marathons -- London, Boston, Chicago, New York, Berlin and Tokyo -- should have faced investigation over potential blood doping following test results -- a quarter of the overall total.
The newspaper did not provide dates for when its statistics allegedly apply or identify the athletes involved.
London Marathon chief executive Nick Bitel said in a statement today that organisers were "very concerned" by the allegations and said the race had a "zero tolerance policy" on doping.
"We believe there are people in our sport who are cheating and everyone has a part to play to protect those who are not," he said.
"We continue to be at the forefront of anti-doping measures for marathon runners as we are determined to make marathon running a safe haven from doping but we cannot do it all on our own and rely heavily on the IAAF."
In a separate development Britain's Mo Farah -- who won Olympic gold medals in 2012 for the 5,000m and 10,000m -- is one of eight athletes who have agreed to release their own blood test data, the newspaper reported.
His coach Alberto Salazar has faced doping allegations but both he and Farah deny any wrongdoing.
"The decision to release my results is a personal one -- I've always said that I'm happy to do what it takes to prove that I'm a clean athlete," he was quoted as saying.
Last week's publication by the Sunday Times and German broadcaster ARD of evidence of hundreds of allegedly suspicious blood tests has drawn a robust response from officials.
The governing body for athletics, the International Association of Athletics Federations (IAAF), has strongly rejected the claims and said the two experts at the centre of the allegations were "naive" and had used "incomplete data".
Sebastian Coe, the former British Olympian and candidate to be president of the IAAF, said last week the claims were "a declaration of war".
The World Anti-Doping Agency said Friday it would "urgently" investigate the allegations.
