It follows Flogas Sunset Cruise collapsing and dying following a practice routine at Cagnes-sur-Mer in France on October 10.

An investigation was immediately opened by the FEI, who claimed the welfare of horses was their "number one priority".

Horse Sport Ireland claimed to be "extremely concerned", while the French Equestrian Federation opened their own investigation.

The Cagnes-sur-Mer Organising Committee filed a report with local police.

"The athlete has been given the opportunity to provide a written reply to the allegations, including any documents, witness statements or other evidence that may be used in any legal proceedings arising out of this case," an FEI statement said today.

"Once this information has been received and a full case file has been compiled, the FEI will submit the matter to the FEI Tribunal under Article 142.2 of the FEI General Regulations.

"As the matter is now the subject of legal proceedings, the FEI will not comment further at this time."

At the time Thornton admitted in a statement that the 10-year-old horse "felt very weird" and "suddenly collapsed".

He admitted to whipping him ""once or twice" during the performance in order to encourage forward movement.

However, he strongly denied suggestions of wrongdoing and said he had been the victim of a "witch-hunt".

"The reactions on the social media of people who do not know the facts and elaborate about the unfortunate incident based on rumours are very disappointing," Thornton said.

"These people do not even identify the correct horse as Flogas, instead referring to Startschuss as the horse that was involved in this tragedy. 

"It escalated into a witch-hunt endangering my safety and reputation which is totally unacceptable. 

"I have received violent threats towards me and abusive messages to myself and my family.

"It is difficult enough to deal with such a tragic incident, but it is even harder to cope with false accusations and violent threats in the press and social media."

Thornton added that he was "very sad and devastated about this tragic incident" and that "it is heart-breaking to see a horse die". 

The 28-year-old is outside the top 1,000 in the FEI rankings list.