Professor Hugh Pennington, president of the Society for General Microbiology and professor emeritus of bacteriology at Aberdeen University, told the Independent the government has greatly underestimated the likely death toll.
Its estimate of 50,000 deaths has already been contradicted by Scotland's chief medical officer, who says it would be 10 times greater.
He criticized the government's "optimistic" attitude to the potentially devastating pandemic -- which experts have warned is both imminent and inevitable -- likening it to complacency over BSE in the early 1990s.
"If the virus moves into people, there will be no stopping it. It will be here before we know it," he said.
On Saturday Vietnamese health officials revealed a 41-year-old nurse caring for a bird flu victim in the country's northern Thai Binh province contracted the disease, increasing fears it is spreading person-to-person.
British ministers admit a pandemic would rapidly reach Britain and have ordered 14.6 million doses of an anti-viral drug. However, they will take up to two years to arrive.