Caster Semenya and other female athletes with naturally high levels of testosterone may be forced to return to taking suppressant medication after a new study found they could have a competitive advantage of up to 4.5 per cent over their rivals.

The double Olympic 800m champion was one of a number of women taking testosterone lowering medication until 2015 when the Court of Arbitration for Sport (CAS) suspended a rule that enforced a limit on female athletes’ naturally occurring levels.

The International Association of Athletics Federations (IAAF) was given two years to respond to that decision and last night published a new report that they believe proves the unfair advantage of high natural levels of testosterone.

The study, funded by the IAAF and World Anti-Doping Agency, and published in the British Journal of Sports Medicine, analysed more than 2,100 androgen samples from athletes participating at the 2011 and 2013 World Championships.

It found females with higher testosterone levels received a significant advantage over athletes with lower levels, with notable increases in 400m (2.7 per cent), 400m hurdles (2.8 per cent), 800m (1.8 per cent), hammer throw (4.5 per cent and pole vault (2.9 per cent).