The Melbourne Cup has again been marred by the death of a horse, this time the Irish-trained The Cliffsofmoher.

The Aidan O'Brien-trained horse suffered a fractured shoulder during Tuesday's race and had to be euthanised, Racing Victoria's executive general manager of integrity services, Jamie Stier, said in a statement.

"The horse received immediate veterinary care, however it was unable to be saved due to the nature of the injury sustained," he said.

Jockey Ryan Moore on The CliffsofMoher is assisted by a race steward after the horse was injured. Photo / AAP

Animal rights group People for the Ethical Treatment of Animals (PETA) has called for an investigation into the death, which it described as "the most recent victim of the cruel annual spectacle".

World Animal Protection also condemned the incident, labelling the Melbourne Cup a "disgrace".

"The Melbourne Cup is the disgrace that stops a nation. It's heartbreaking to see another horse needlessly lose its life during the Melbourne Cup," senior campaign manager Ben Pearson said in a statement.

The Cliffsofmoher becomes the 6th horse to die as a result of the #melbournecup since 2013. Highlights the very real risks to horses from racing.

— RSPCA Australia (@RSPCA) November 6, 2018

"World Animal Protection opposes the use of all animals in entertainment, including the use of horses in the racing industry. It's time to end the cruelty."

Social media was flooded with angry messages from people condemning the racing industry in response to what veteran commentator Bruce McAvaney described as a "tragedy".

Beat your dog in the street and you'll be, quite rightly, arrested. Beat a horse to death around a racetrack and you're part of the Australian cultural identity. It's a travesty. #MelbourneCup2018

— Alan Baxter (@AlanBaxter) November 6, 2018

Did another horse die so we could gamble? #MelbourneCupDay

— Anthony Hayes (@theanthonyhayes) November 6, 2018

O'Brien called the death "very sad" but said it was lucky jockey Ryan Moore remained unharmed.

"Unfortunately these things can happen to a horse galloping around the field at home," O'Brien said.

"It's very sad. It could have been worse, Ryan could have taken a fall off him, someone could have been seriously injured."

It is the fourth time in the past six years that the Cup has been shrouded by the death of horses.

Another horses dies in the Melbourne Cup.. Poor thing broke its leg.. So they killed it on the race track.. Enjoy that imagine while you sip on your champagne... #NupToTheCup

— Smallzy 🎧 (@Smallzy) November 6, 2018

French-trained horse Verema had to be put down after an injury suffered in the 2013 Cup.

A year later pre-race favourite Admire Ratki died after collapsing from heart failure in the stalls minutes after the race.

In the same 2014 race, import Araldo had to be euthanised because of an injury suffered when he was returning to scale.

Do we really need to watch animals be slaughtered for our own entertainment? Maybe it’s time horse racing went the way of Spanish bullfighting. RIP Cliffsofmoher 😢 #NuptotheCup

— Jason Ball (@jasonballau) November 6, 2018