Brazilian Olympian Carlos Parro has been issued a warning after performing a controversial practice that restricts oxygen to horses' jaws and turns their tongues blue.

Equestrian's governing body, the Federation for Equestrian Sports (FEI), has cautioned eventer Parro for causing "unnecessary discomfort to a horse" as concerns regarding the animal continue to grow after the scandal involving Brit Charlotte Dujardin.

FEI president Ingmar De Vos received a letter from animal rights group PETA with evidence of Parro allegedly hyperflexing Safira's neck in a banned movement called 'Rollkur', where a horse's breathing is harmed.

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"The FEI Officials have issued a yellow warning card (to Parro) as the action of the athlete could have caused unnecessary discomfort to the horse. Additionally, the Ground Jury has directly addressed the situation with the athlete and the chef d'Equipe of Brazil," the federation's statement read.

"The FEI Stewards are aware of this incident and will be vigilant at all times for any behaviour that is not in line with horse welfare and are prepared to take immediate action if such behaviour occurs."

Parro, 45, competed in the dressage section of eventing at the Olympics on Saturday morning, before a warning was issued against the competitor. PETA confirmed that they had sent their letter on Friday evening.

PETA's letter explained that 'Rollkur' also damages horses' spines and can cause long-term health issues. The organisation has doubled down on their call for the Olympic Games to drop all equestrian events.

"The pain this horse must be enduring is almost unimaginable," PETA said. "The public is unlikely to accept this, as illustrated by the mass outrage over the Charlotte Dujardin abuse video.

"The hyperflexion of a horse’s neck through aggressive force, known as 'rollkur' is banned at international equestrian events. Rollkur is extremely painful and can block a horse’s airway, damage the spine, and cause long-term health problems.

"This latest instance of horse abuse comes just days after a video surfaced showing British Olympic equestrian team member Charlotte Dujardin repeatedly striking a horse during a training session."

A dark cloud has been cast over horse welfare after three-time British Olympic gold medalist Dujardin was provisionally suspended from all events, pending an investigation, after a video emerged of her repeatedly whipping a horse.