LONDON - Hundreds of thousands of people may die and one quarter of the work force could be absent if Britain were hit by a bird flu pandemic, a senior government official said.
"It may be somewhere between 20,000 and 750,000 extra deaths and it may be 25 percent of the population off work," the government official, speaking on a non-attributable basis, told a conference in London.
"That is the shape of the event we are going to have to deal with," he said.
Britain's population is nearly 60 million people, with 28 million working, according to government figures.
Contingency plans already announced by Britain's health department include the stockpiling of the anti-viral drug Tamiflu at a cost of 200 million pounds (380 million dollars, 290 million euros).
The country's chief medical officer, Sir Liam Donaldson, has also previously described a national preparedness plan the government is ready to put in place should the deadly H5N1 strain of the bird flu virus develop into a new strain that spreads from human to human.
Measures include closing schools and cancelling public gatherings like football matches and pop concerts, as well as issuing travel warnings.
The estimate of 750,000 dead put forward was described later Tuesday by a health department spokeswoman as a "theoretical upper limit" of a catastrophe.
She said the government was sticking to its estimate of 50,000 British deaths, a number advanced earlier this month when it published its contingency plan.
The higher figure, presented to an international forum at the International Institute for Strategic Studies came days, came days after a leading scientist warned that the government's estimate was "optimistic".
Professor Hugh Pennington, president of the Society for General Microbiology, said he believed up to two million Britons could perish from a mutated form of the H5N1 virus.
He has criticised current planning for an outbreak, warning that a strain affecting humans will be "here before we know it".
Though the government has ordered 14.6 million vaccine doses for Britain they will take up to two years to arrive, prompting some worries that the population could be at risk in the interim.
Since last January, some 46 people in southeast Asia, most of them in Vietnam, have died after contracting a type of the disease as a result of contact with sick or dead birds.
Medical experts have warned that if the virus develops the ability to pass from human to human, the consequences would be devastating.