Five horses have died due to neglect in a field in Leighton Buzzard, Bedfordshire.
Local residents and passers-by have been shocked at the state of the group of horses being kept on land in Grovebury Road for some time.
Five horses have now died, but nine remain on the land.
The RSPCA said it “shares serious concerns” and is checking the horses on a daily basis.
“We brought in a vet to examine and treat them [earlier this month] and he found some of the horses had cyathostomes (small redworm),” said a spokesman for the charity.
“Three horses very sadly died at the site as a result of this, and four others were removed by us after the vet said they needed urgent help. Very sadly two had to be put to sleep as they were so unwell.”
The RSPCA said it is treating the situation “very seriously”.
“We are looking into several ways of providing a long-term solution to ensure the welfare of the remaining nine horses, but will continue to keep a close eye on them to make sure they are not at immediate risk in the meantime,” added the spokesman.
“We have attempted to contact their owners to improve the conditions they are being kept in and the care that they receive, but have as yet been unable to locate them.”
The RSPCA added that it has no legal power to remove the animal without permission of the owner and that the country is in a grip of a “horse crisis”.
Yesterday (Thursday 8 January) a new bill to clamp down on fly-grazing moved a step closer to becoming law.
The Control of Horses Bill, tabled by the MP for York Outer, Julian Sturdy, has passed the committee stage and now goes to its final stage in the House of Commons.
Currently farmers and landowners, local authorities, animal welfare charities and vets have few powers to tackle fly-grazing because action depends on identifying owners who are absent.
The Bill aims to help England follow in the footsteps of Wales, which introduced legislation in January 2014 giving local authorities the power to seize or impound horses abandoned on public or private land without permission.
“Until now the legal powers to address the problem have been lacking but if this Bill is passed it will be a significant step forward for animal welfare and landowners’ rights,” said Countryside Alliance executive chairman Barney White-Spunner.