Cases were opened against Germany's Felix Etzel and Lebanon's Emile Karim Fares after horses they rode in May 2017 tested positive for the substance.

Etzel rode Inception 2 at an event at Wiener Neustadt in Austria while Fares rode Finest Quality V&K at a competition in Eindhoven.

The FEI said they have been told the presence of the substance can be the result of contamination due to the ingestion of the flower colchicum autumnale.

The flower is commonly known as autumn crocus.

Demecolcine was previously listed as a banned substance by the FEI, but it was reclassified to a "specified" substance on January 1.

Specified substances are considered more likely to have been ingested by horses for a purpose other than the enhancement of performance, including through a contaminated food substance.

In cases involving a specified substance, provisional suspensions are not automatic.

Demecolcine is not pharmaceutical, but is used in human medicine in tumour therapy.

The FEI said there is no known use for demecolcine in veterinary medicine, with alkaloids of the autumn crocus all extremely toxic.

Both athletes were able to prove to that the substance had entered their horses’ systems through ingesting hay that had been contaminated by autumn crocus.

They were also able to establish that they bore no fault or negligence for the rule violation.

The FEI Tribunal ruled that no further sanctions should be imposed, other than the automatic disqualification of the horse and athlete from the competitions.