UK Anti-Doping (Ukad) has begun an investigation and expressed "grave concern" over claims by two Kenyan doctors they gave banned performance-enhancing drugs to athletes from Britain and elsewhere.

The medics were secretly filmed by the Sunday Times, which has investigated allegations of doping in the African country in collaboration with German broadcaster ARD/WRD.

In a statement, UK Athletics described the Sunday Times' allegations as "vague and unsubstantiated" and encouraged the newspaper to "provide full details of all of the individuals and information in its article as a matter of urgency so that the claims can be investigated in a timely manner".

Laura Trant reports.