Opponents of a planned levy on horses in Britain under cost-sharing plans for animal health fear the "tax" could top £100 per equine.
The levy has been proposed by the Department for Environment, Food and Rural Affairs (Defra) as part of a new structure to fund animal disease control.
The heads of major horse group in Britain have urged the government to rethink its plans.
Industry leaders put their signatures to a letter which appeared in The Times of London.
They said the levy was a tax in all but name and the plans would impose a £50-million-plus animal health quango (quasi non-governmental organisation) on all horse owners in Britain.
"The Government's responsibility and cost-sharing proposals would serve to shift responsibility for managing outbreaks of animal disease away from ministers, breaking the vital link between health and welfare, while hitting horse owners with a new estimated charge of £10.50 per horse each year and perhaps as much as ten times this amount," they said.
"The initial additional administrative cost to the taxpayer will be £14.3 million.
"At a time of recession and cutbacks in public expenditure, it is an unnecessary use of public money to create a complex new layer of government administration" they said.
It would cause severe fragmentation in the delivery of essential animal health and welfare services when existing, less expensive solutions could be maintained."
The expenditure involved would bring no new benefit for horse health and welfare, they said.
"To rural communities, horse owners in the cities or countryside, and to equine businesses that are already struggling through difficult economic times, a significant charge for absolutely no benefit to fund a new quango is yet another unwarranted and unfair burden.
"We hope the Government will think again."
The signatories to the letter said they represented millions of people who worked with and rode horses. They were: Andrew Finding (British Equestrian Federation); Claire Williams (British Equestrian Trade Association); Chris House (The British Equine Veterinary Association); Graham Cory (British Horse Society); Professor Tim Morris (British Horseracing Authority); Louise Kemble (Thoroughbred Breeders' Association); Rupert Arnold (National Trainers Federation); Roly Owers (World Horse Welfare); Terry Court (Welsh Pony and Cob Society); and Michael Harris (Racehorse Owners Association).
While the original proposal from Defra suggested an annual levy of around £10.50, its opponents fear set-up costs and expenses involved in collection could see the levy reach 10 times the proposed amount.
The British Horse Industry Confederation, which has made a submission to Defra on the levy, said the proposals were based on inaccurate data and flawed assumptions.
The proposed levy represented an increase in costs for the equine sector where a significant proportion already fund their leisure activities out of taxed income, it said.
It pointed to the significant social, cultural and commercial impact in Britain of horses, ponies and donkeys.
"Our understanding of the core of the Government's proposals is that horses will be included in a new independent body with industry representation, separate from Defra, that will manage animal health, and that costs will be charged in proportion to the value of sector output.
"The horse sector has direct, core expenditure of £5 billion and when secondary expenditure is included it brings the total economic impact to over £7 billion.
"It is the largest sporting employer in the country."
It continued: "The horse sector is not a smaller version of the livestock sector. Instead it has a significant primary non-commercial structure, creates significant output through expenditure of taxed income, and receives no public production subsidy.
"We are not complacent, and recognise that more can be done on biosecurity and disease surveillance, both by us and Government.
"We are ready to change and look at innovative ideas ... we are a willing partner of Government.
"The proposals for responsibility and cost-sharing are based on inaccurate data and flawed assumptions, represent a increase in costs for a sector where a significant proportion already fund their leisure activities out of taxed income.
"We have seen no information on objective measurement of efficiency and accountability. The proposals are unworkable and of no benefit and must not proceed in their present form.
"The BHIC recommends the Government fundamentally reviews its proposals to bring forward a well-reasoned set of initiatives, assuming that enforcement is clearly undertaken, which is targeted at the sector's needs and structure, and not an inappropriate add-on to a farm livestock scheme."