Conservation chiefs say footpaths should stay shut despite warnings it could cripple the county's tourism industry this summer.
Sussex Downs Conservation Board has warned councils to use "great caution" when considering any relaxation of the ban on using footpaths and bridleways as the battle to keep the county free from foot-and-mouth disease continues.
Its view is at loggerheads with the Government's bid to lure foreign visitors back to the UK.
Ministers, who claim there is no evidence the disease is spread by tourists and ramblers, want restrictions lifted in disease-free areas.
Roger Marlowe, chairman of Brighton and Hove Hotels Association, said it was crucial action was taken to protect tourism, which he said was worth more to Sussex than agriculture.
He said: "The situation is totally serious. I know of at least one American couple who cancelled a walking holiday in England later this year in favour of Brittany because of foot- and-mouth. We are affected and if the Government is saying the council can relax the rules then it absolutely must.
"It is very important to protect the tourism industry.
"At the moment Brighton still isn't suffering because most of our visitors are from the UK, but we will in the summer when we normally get an influx of American and European visitors.
"Everyone will be affected, the hoteliers, the restaurants and bars, these places will all be empty and the city will lose a lot of money."
The Sussex Downs Conservation Board has backed last week's decisions by East and West Sussex County Councils to keep footpaths closed.
It said it recognised the needs of the tourist industry and the frustrations of walkers and riders but said farmers should continue to be supported.
Countryside manager Phil Belden said: "We are constantly assessing the risks of opening up more safe sites for walkers and riders, however, our primary objective must be to keep foot-and-mouth disease out of the county."
Brighton and Hove is among the ten most popular UK destinations for overseas visitors, with 300,000 staying in the city each year. Many more choose it for day trips.
The call to reopen the countryside was supported by Kemp Town MP Des Turner, who said: "Unless footpaths might bring people into close proximity to livestock, people should be able to walk."
But Michael Fordham, chairman of the National Farmers' Union in East Sussex, said: "The problem is you have to open all or none of the footpaths. Sussex is disease free and we wish it to remain so."
The board also said it would be "wrong and risky" to bury carcasses of animals killed during the emergency at landfill sites in the county as potentially infected lorries and animals could be brought into Sussex.
More than 2,600 sheep were buried at the Brookhurst Wood landfill site at Warnham, near Horsham. Operator Biffa Waste Services has suspended the operation because of concerns from people who live nearby.
Nationally, 1,108 cases of the disease have been confirmed and it will be at least a week before experts can predict if the outbreak is slowing.
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