A still from the video that sparked the Rollkur debate: Swedish rider Patrick Kittel rides the stallion Watermill Scandic, at Odense in October.
Kittel was criticised by welfare organisations for his riding of Watermill Scandic, who appeared to show signs of distress in video footage taken at the event.
The FEI said it has reviewed witness statements and video evidence, "and the investigation findings are that there is no reliable evidence that the warm-up techniques used by Mr Kittel were excessive."
The FEI's Legal Department has ruled that no formal claim against Kittel will be submitted to the FEI Tribunal.
Kittel, however, has received a warning letter "regarding the appropriate and inappropriate use of warm-up techniques and has been made aware that his actions will be watched very carefully going forward to ensure that there are no subsequent violations of FEI rules relating to horse welfare," the FEI said.
The organisation is conducting a major review and analysis of its current policy on pre-competition warm-up activities, with an emphasis on hyperflexion.
At the time the investigation into Patrick Kittel was announced last November, FEI secretary-general and chief executive Alexander McLin acknowleged the matter had attracted much attention.
"A video featuring an international rider using Rollkur or hyperflexion - a questionable and potentially abusive training method ... was posted on YouTube," McLin said.
He noted that the FEI had received numerous emails on the video, which had received thousands of views and generated strong debate on social media on the issue of animal welfare.
"FEI's main concern has always been and will always be the welfare of the horse," he said.