British athletes training at the Kenyan town of Iten are among those claimed to have been using performance-enhancing drugs, similar to those used by disgraced cyclist Lance Armstrong. Up to 50 athletes in training in the town – used by athletes because of its high altitude – are said to have been given injections of erythropoietin (EPO) by Kenyan doctors.

The claims are being made following an investigation made by the Sunday Times and German TV station ARD. In undercover filming, two doctors and their associate said they had provided the drug to at least four Britons, as well as Kenyans and those of other nationalities. The doctors then produced medical notes and blood test results so it would appear they had simply treated the athletes for injuries.

When formally approached by the Sunday Times the doctors denied their former claims. But officials from UK Anti-Doping (UKAD) are now in Kenya. Nicole Sapstead, UKAD chief executive, said: "UK Anti-Doping has reviewed the evidence presented to us by The Sunday Times and it is of grave concern and of significant interest. We have opened an investigation and are taking the neces­sary steps to corroborate the evidence and investigate it further."

The UK athletics governing body said the reports were "vague and unsubstantiated" and more evidence would be required. "None of the allegations as presented relating to British athletes accords with our experience," a representative told Sky Sports. "Nevertheless, we take any allegations of doping seriously and will as always cooperate fully with any investigation undertaken by UKAD or other anti-doping organisations."

The German documentary includes footage apparently filmed at the High Performance Training Centre in Iten, showing boxes of EPO and syringes are seen in bins. The owners of the centre, Pieter Langerhorst and wife Lornah Kiplagat, said they maintained a zero tolerance policy towards illegal drugs.

"We have installed 16 cameras to monitor the camp," Pieter Langerhorst told The Observer. "If we have any concerns about athletes, we immediately report the athlete to the IAAF who will normally test the athlete that same day. We have no refrigerators in any of the rooms so no one can keep EPO in their rooms."

What is EPO?

Erythropoietin (EPO) is a natural hormone made by the kidneys, which triggers the generation of oxygen-carrying red blood cells. EPO injections trigger the creation of more blood cells, which boost an athlete's endurance levels. Studies have shown a four-week course of EPO can increase in an athlete's performance to a point that would take years of training. Conventional drug tests cannot detect the use of EPO, but new tests have been developed.