The cull of 4,000 sheep starts in the Brecon Beacons in Wales today in a bid to prevent foot-and-mouth disease from spreading across the national park.
Blood tests on sheep in five areas of the mountain range showed a significant number of positive results.
Welsh Rural Affairs Minister Mr Carwyn Jones announced the slaughter programme on Wednesday, sparking fears that large numbers of cases could still be lying undetected across the country.
Tourism chiefs predicted the mass cull would "devastate" business in the mid-Wales area.
Chief executive of Mid Wales Tourism Mr David Peate said foot-and-mouth has already cost tourism operators in south Powys about £50 million sterling and said he feared many will not survive.
A spokesman for the British Department for Environment, Food and Rural Affairs said: "We have already tested many tens of thousands of sheep and have discovered no signs of the disease outside areas where we already knew it existed.
"It is not a surprise to find the disease on the Brecon Beacons because there have been confirmed cases nearby. If a lot of cases were confirmed in areas where the disease has not been found, that would be a different matter. But that has not happened".
The blood tests showed antibodies in the sheep and not the presence of full-blown foot-and-mouth disease, Welsh Assembly officials said.
PA