If Britain fails to control foot-and-mouth, 50% of livestock will have to be destroyed, the Government's chief scientist has warned.
Prof David King's comments came as the number of outbreaks reached 501, with 21 new cases today.
Tony Blair has ordered a nationwide extension of a "firebreak" cull of healthy animals, next to infected farms, to control the crisis.
A MAFF report says the crisis is expected to continue for months and become "a large epidemic".
It estimates that more than 4,400 cases of foot-and-mouth would be recorded by June.
At present the "firebreak" cull is only being used in two-mile areas around infected farms in Cumbria and Dumfries and Galloway.
A Ministry of Agriculture (MAFF) spokeswoman said the practicalities of employing the policy nationwide still had to be worked out. However, the proposals immediately received support from National Farmers' Union president Ben Gill.
Mr Blair also instructed officials to improve slaughtering times to ensure that all livestock is culled within 24 hours of diagnosis, to avoid spreading the virus.
As it was disclosed that more than 480,000 animals had been slaughtered or condemned, the current outbreak has been officially declared worse than that of 1967.
Professor King told the BBC: "Today's models show that if we proceed as we are at the moment, the epidemic is out of control. In the worst-case scenario, out of control means that we might even lose 50% of the livestock of Great Britain."