Alberto Salazar has been accused of using prohibited drug infusions to improve the performance of his runners, according to a report.

Leaked United States Anti-Doping Agency documents reportedly seen by the Sunday Times are said to show that double Olympic champion Sir Mo Farah and other athletes were given infusions of a research supplement based on the chemical L-carnitine.

The naturally-produced amino acid, which is normally prescribed as a supplement for heart and muscle disorders, is not a banned substance for athletes, but infusions of more than 50 millilitres in the space of six hours are forbidden.

The leaked report also alleges that Salazar, head coach of the world famous endurance Nike Oregon Project (NOP), routinely gave Farah and other athletes legal prescription drugs with potentially harmful side-effects without a justifiable medical reason.

In a statement, USADA said it could "confirm that it has prepared a report in response to a subpoena from a state medical licensing body regarding care given by a physician to athletes associated with the Nike Oregon Project.

"We understand that the licensing body is still deciding its case and as we continue to investigate whether anti-doping rules were broken, no further comment will be made at this time.

"Importantly, all athletes, coaches and others under the jurisdiction of the World Anti-Doping Code are innocent and presumed to have complied with the rules unless and until the established anti-doping process declares otherwise. It is unfair and reckless to state, infer or imply differently."

Salazar and Great Britain long-distance runner Farah deny that they have ever broken anti-doping rules.