Brown said inquiries were continuing to pinpoint the cause of the outbreak, but acknowledged the disease strain found in two infected cattle is the same used at a research laboratory about 4km from the scene of the outbreak.
The European Commission has backed Britain's self-imposed export ban.
The spokesman for the European commissioner for argriculture, Philip Tod, said: "The British authorities have informed us that they wish the whole of Great Britain to be treated as a high risk area, as a precaution as I said, to allow them to complete the tracing of animal movements."
The last outbreak of foot and mouth disease in 2001 lead to the culling of six-and-a-half to 10 million animals, and cost British agriculture up to $17bn.
The nearby town of Guildford is already counting the cost: its farmers' market, held monthly has been cancelled.
Andrew Hodges, of Guilford council, said: "[It is] one of the largest markets around. It's very popular, it's something that will be missed."
Livestock samples were being tested from 100 different premises, but there have been no new cases.
The US and Japan placed a ban on British pigs and pork products on Saturday, adding to an existing ban on British beef in both countries because of Bovine Spongiform Encephalopathy (BSE), also known as mad cow disease.
The US department of agriculture said that it was barring all products derived from foot and mouth-susceptible species in Britain.
Foot and mouth disease causes high fevers and blisters in cloven-hoofed animals and can often lead to death.
It can be contracted by cattle, pigs, sheep and goats, and very rarely by people.
The disease spreads fairly easily as it can be carried on the wheels of vehicles, in livestock units, on shoes and even on the wind, officials said.