Successful racehorses linked to some of the biggest names in racing were among thousands slaughtered in British and Irish abattoirs since 2019.
That is according to a BBC Panorama documentary, which on Monday showed footage filmed inside one of the UK's biggest abattoirs.
Secret recordings also claimed that rules designed to protect horses from a cruel death were being breached.
The BBC says Freedom of Information requests revealed that 4,000 former racehorses were slaughtered in Ireland and Britain since the beginning of 2019.
Most - but not all - were trained in Ireland, it says.
The footage was recorded over four days at the end of 2019 and the start of 2020.
Founder of animal rights group ARAN, John Carmody, told Newstalk Breakfast this did not surprise him.
"This is something that we would have been aware of for a hell of a long time - we've been screaming from the rooftops to people who place bets and bookies around the country for years, telling them that this was inherent in that industry, and thankfully viewers got a look at that last night.
"Wherever animals are used for entertainment and profit corners will always be cut.
"The reality here is this: anyone that goes in and places a bet on horseracing, greyhound racing or any other animal that has to run for its life, there is going to be huge problems with the welfare of these animals.
"There's always a downward spiral of animals who end up needing to be killed - and I think viewers up until last night may have heard it from time to time... but very few people would have believed that this was actually happening in abattoirs and in slaughterhouses."
John says this has come to light not by animal rights groups, but the BBC.
"The reality is that these animals are being killed in their thousands, and it didn't come from animal rights people - it came from the BBC Panorama last night".
In a statement, Horse Racing Ireland says it unreservedly condemns the practices shown in the images, and supports calls for an investigation.
"Horse Racing Ireland unreservedly condemn the practises show in the images from the Swindon, UK abattoir, portrayed this evening in the BBC documentary Panorama.
"HRI support calls for an investigation by the relevant UK authority into this abattoir.
"This footage showed both animal and human health issues and is not reflective of the care that racehorses receive in the horseracing industry throughout their lives", it says.