Horse racing: Fallen Melbourne Cup day horse Regal Monarch put down

Jockey Joao Moreira lays on the ground after Regal Monarch fell in race 4. Photo / Getty

Racehorse Regal Monarch has been euthanised after falling at Flemington on Melbourne Cup day.

The horse was taken to Werribee Veterinary Hospital after the fall in the fourth race on Tuesday, but could not be saved, Racing Victoria's chief vet Grace 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," she said.

"Our sympathies are extended at this time to the owners of Regal Monarch, trainer Chris Waller, and his staff who cared deeply for their horse and are naturally devastated by their loss."

Jockey Joao Moreira was cleared of serious injury but was unable to ride in the main race.

He hit the ground when the Chris Waller-trained Regal Monarch came down near the 300m of the Ronald McDonald Plate (280m).

The champion Hong Kong jockey limped back to the jockeys' room after the fall and was stood down from his remaining rides.

Apprentice Ben Allen picked up the ride on Irish horse Thomas Hobson in the main event.

The lightweight Allen was the only rider in the jockeys' room able to ride the Willie Mullins-trained stayer at 52kg.

Thomas Hobson, one of three runners for Mullins, finished sixth. - AAP