The Court of Arbitration for Sport (CAS) on Monday ruled against American Paralympian Blake Leeper's attempt to compete in next year's Tokyo Olympics wearing prosthetic legs.

The 31-year-old Leeper, who was born without lower legs, won silver in the 400 and bronze in the 200m in the T43 class at the Paralympics in London in 2012 but wanted to emulate  South African Oscar Pistorius, nicknamed 'the Blade Runner', who competed in the Olympics that year.

Leeper was appealing a decision by World Athletics (formerly the IAAF) in February to ban him from competing with his blade-shaped prostheses. 

While CAS said in its ruling that it upheld Leeper's argument that the World Athletics (WA) rule was "unlawful and invalid", it also decided that the prosthetics gave him a "competitive advantage."

Leeper's lawyer reacted to the ruling by accusing CAS of racism.

CAS said it was unfair that WA's rule "places the burden of proof upon an athlete" but it agreed with the intention to allow disabled athletes to compete without using "mechanical aids that do more than compensate for the effect of their disability."

After hearing expert witnesses, CAS said it had "concluded that the running-specific prostheses used by Blake Leeper indeed gave him an overall competitive advantage in the 400m event over an athlete not using such a mechanical aid since they enabled him to run at a height that was several inches taller than his maximum possible height if he had intact biological legs."

Leeper broke Pistorius's world record in the T43 400m in 2017.

The WA ban last February prevented Leeper from running at the World Athletics Championships in Doha in October 2019 even though he finished fifth in the US championships and would have made the relay squad. 

His best time was 44.38sec in the semifinals, well inside his own T43 record, but 1.25sec slower than Wayde van Niekerk's world record and 0.9sec slower than Bahamian Steven Gardiner's winning time in Doha.

"World Athletics welcomes CAS's ruling that on the facts of this case," the organisation wrote in a statement. 

Leeper's lawyer Jeffrey Kessler was unhappy CAS had accepted their challenge on principal but still found against the runner.

Kessler also suggested the height data used was racist telling the Inside the Games website CAS was "authorising an even more insidious form of racial discrimination - against black disabled athletes - by finding that their prosthetic limbs are subject to artificial height limitations based solely on the body proportions of Caucasian and Asian athletes."

Kessler said he planned to take the case to civil court.

WA responded that it rejected "the new allegation from Mr Leeper's legal team that the finding that he 'runs tall' is based on a 'racist' Paralympic rule."

"The 15cm disparity found in Mr Leeper's case between his prosthetic leg length and his natural leg length is not due to racial differences in body dimensions."

The CAS decision means in future it will be up to the federation to prove "equipment" does not give athletes an advantage.