He reportedly suffered a serious neck injury.
The renowned amateur rider was partnering Galaxy Rock in the Fulke Walwyn Kim Muir Challenge Cup when the pair crashed out at the first fence. Medical staff were quickly on hand to treat McNamara and it was decided to transfer him to Frenchay Hospital in Bristol for further treatment.
Course spokesman Andy Clifton said: “JT McNamara is being transferred to Frenchay Hospital in Bristol via air ambulance and continues to undergo assessment.”
The rider was reported to be conscious following the fall, but McNamara’s uncle, trainer Andrew McNamara, later said JT was put in an induced coma.