Paul O'Neill will face hearing next week and could face a lengthy suspension after the organization's disciplinary department studied tapes of the incident Monday and found that the jockey's attack on City Affair violated rules, The Times of London reported Tuesday.
David Muir, equine consultant for the Royal Society for the Prevention of Cruelty to Animals, described O'Neill's head butt as "stupid and juvenile."
"We are satisfied that the horse was not injured but it was an unacceptable way for the jockey to behave. We'll see what action the HRA take before looking at whether we might bring a case against him ourselves," he said.
O'Neill, who rides jumping horses owned by Paul Beck and some partially owned by Andrew Flintoff, is also involved in an investigation into irregular betting patterns surrounding a horse ridden by the jockey in March.