The horse cruelty scandal at the Australian Turf Club has escalated after the nation's peak animal welfare body announced its own independent investigation into claims of mistreatment.

The ATC has been rocked by allegations that the stable of ex-racehorses that make up its mounted security division were subjected to a "deliberate program" of drugging by staff.

The ATC's security horses featured in last year's series of Australia's Next Top Model. Credit:FOX8

Aside from ceremonial duties on race days, the retired thoroughbreds are advertised as horses for hire at festivals, private functions and shoots on television shows such as Australia's Next Top Model.

But according to evidence aired at a Racing NSW inquiry, the horses were sedated through much of their work with drugs such as Sedazine paste and liquid acepromazine (ACE) – an injectable sedative that can only be legally given by a vet.