The South African 800m Olympic champion Caster Semenya is considering an appeal after losing her landmark legal case against athletics’ governing body, the IAAF, in a decision that could end her career as an elite athlete.

The ruling by the court of arbitration for Sport means that Semenya, who has not been beaten over 800m since 2015, will have to take medication to significantly reduce her testosterone if she wants to run internationally at events between 400m and a mile.

How the Caster Semenya controversy has unfolded since 2009 – a timeline Read more

The sports scientist Ross Tucker, who was part of Semenya’s team of experts at Cas, believes it will mean the South African could run the 800m in around seven seconds slower – turning her from a world-beater into an also-ran at that event. However the indications are that she may decide to step up to the 5,000m, where the IAAF’s new rules regarding athletes with differences in sex development (DSDs) do not apply.

The surprise verdict was announced by the court of arbitration for sport on Wednesday after three arbitrators had spent more than two months deliberating over the complex and highly contentious case.

Announcing its ruling, Cas agreed that the IAAF’s policy was “discriminatory” to athletes with differences in sexual development (DSDs) such as Semenya. However two of three arbitrators accepted the IAAF’s argument that high testosterone in female athletes confers significant advantages in size, strength and power from puberty onwards, and said the policy was “necessary, reasonable and proportionate” to ensure fair competition in women’s sport.

It means that all DSD athletes, who are usually born with internal testes, will have to reduce their testosterone to below five nmol/L for at least six months if they want to compete internationally all distances from 400m to a mile. The IAAF, which welcomed the news, said its policy would come into place on 8 May.

Semenya, who has long argued that her unique genetic gifts should be celebrated not regulated, said she would not give up her fight and believed that the DSD regulations would be one day overturned.

“I know that the IAAF’s regulations have always targeted me specifically,” she added. “For a decade the IAAF has tried to slow me down, but this has actually made me stronger. The decision of the Cas will not hold me back. I will once again rise above and continue to inspire young women and athletes in South Africa and around the world.”

Sign up to The Recap, our weekly email of editors’ picks.

Explaining its verdict, Cas said that Semenya’s team had been unable to prove the IAAF’s policy was “invalid” during the five-day tribunal in February. Crucially it upheld a rule that discrimination in sport is legal provided it is justified.

As it explained in a statement: “The panel found that the DSD regulations are discriminatory but that, on the basis of the evidence submitted by the parties, such discrimination is a necessary, reasonable and proportionate means of achieving the IAAF’s aim of preserving the integrity of female athletics in the restricted events.”

However, in its 165-page ruling, the Cas panel expressed “some serious concerns” as to whether the DSD regulations would be applied fairly. It also accepted there could be problems in three other areas:

• Difficulties for athletes in implementing and complying with the DSD regulations due to having to frequently take medication

• The absence of concrete evidence supporting the inclusion of certain events under the DSD regulations, including the 1500m and one mile

• The potentially negative and harmful side-effects of hormone treatment for DSD athletes.

On the health issues, Cas went even further, saying that further research into the effects of hormone treatment could “demonstrate the practical impossibility of compliance which could, in turn, lead to a different conclusion as to the proportionality of the DSD regulations”.

A spokesperson for Semenya urged the IAAF to take heed of Cas’s concerns and postpone its plan to introduce the DSD regulations next week. “Ms Semenya believes that the dissenting Cas arbitrator will be shown to be correct and the DSD regulations will be overturned,” the spokesperson added.

“In the interim, Ms Semenya believes that it is irresponsible for the IAAF to proceed with the implementation of the DSD regulations in circumstances where the Cas decision makes it abundantly clear that there are serious problems with the regulations.”

Semenya had taken the IAAF to court arguing its policy was discriminatory, unfair, and potentially posed a health risk. However during the five-day hearing last month, the IAAF maintained that its policy was solely about creating a level playing field for all woman, so that success was based on talent and hard work. A key part of its argument was that more than 99% of females have around 0.12-1.79 nm3ol/L of testosterone in their bodies – while DSDs like Semenya are in the male range of 7.7-29.4 nmol/L.

I was sore about losing to Caster Semenya. But this decision against her is wrong | Madeleine Pape Read more

The IAAF’s view was later backed by the leading sports scientist John Brewer, the professor of applied sports science of Buckinghamshire New University. “While this ruling is a sad outcome for Caster Semenya it is not unexpected,” he added. “Testosterone is a growth hormone which is why it appears on the World Anti-Doping Agency’s list of banned drugs, and we know that at high levels testosterone is performance enhancing.

“Clearly nobody wants to discriminate against any individual who wants to take part in sport. But without this ruling we would have been in a position where females with normal levels of testosterone would be at a performance disadvantage compared with those who have higher levels of testosterone.”

Athletics South Africa, which had strongly backed Semenya during her case, said it was “reeling in shock at the how a body held in high esteem like Cas could endorse discrimination without flinching.”

“Cas has seen it fit to open the wounds of apartheid – a system of discrimination condemned by the whole world as a crime against humanity,” it added. “For Cas not to only condone discrimination but also go to lengths to justify it, only undermines the integrity that this body is entrusted with. We believe their decision is disgraceful.”