Horror fall rocks Melbourne Cup Day

Shockwaves swept through the Melbourne Cup carnival at Flemington after a horror accident in race 4.

Jockey Joao Moreira, who was riding international raider Thomas Hobson in the main race, suffered a nasty tumble on Regal Monarch.

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The incident happened just before the straight as the Chris Waller-trained five-year-old gelding seemed to get in a tangle with the pack and went crashing to the turf.

The horse was able to walk onto the float before being transported to Werribee Veterinary Hospital, but it was later 'humanely euthanised'.

Regal Monarch was taken to Werribee equine for treatment. Getty

Racing Victoria's chief vet Dr Grace Forbes revealed on Tuesday night that despite the best efforts of leading equine vets, the Chris Waller trained horse was unable to be successfully treated.

"Regal Monarch was immediately attended to by our veterinary team after a fall in Race 4 and was transported to the University of Melbourne Veterinary Clinic in Werribee for further assessment," Dr Forbes said.

"It is with sadness that we can confirm that the horse has been humanely euthanised on veterinary advice as a result of the injuries sustained. This decision was made in the best interests of the horse."

Moreira was visibly limping but managed to give the crowd a smile as he walked back off the track.

However, in a massive blow for the jockey and connections to Thomas Hobson, the injury ruled Moreira out of action for the remainder of the day.

The incident left a big question mark over who would pilot Thomas Hobson in the Melbourne Cup with 18-year-old Ben Allen getting the nod.