Two million Britons could die in the bird flu pandemic that experts warn is both imminent and inevitable, one of the country's leading authorities has told The Independent on Sunday.

Professor Hugh Pennington, the president of the Society for General Microbiology and professor emeritus of bacteriology at Aberdeen University, also criticised the the Government's "optimistic" attitude to a potentially devastating pandemic, likening it to official complacency over BSE a decade ago.

In the starkest warning yet over the potentially devastating impact of the pandemic, Professor Pennington said that the number of deaths has been greatly underestimated. He expects the flu - like the 1918 pandemic which killed more people than the First World War - to cause the deaths of many people from pneumonia "which we are still not very good at treating".

He said: "If the virus moves into people there will be no stopping it. It will be here before we know it." Ministers have sought to play down the potential impact of bird flu by saying that only some 50,000 people would die in Britain. But this has already been contradicted by Scotland's chief medical officer, who says it would be 10 times worse.

Yesterday Vietnamese health officials revealed that a 41-year-old nurse who had cared for a bird flu victim in the country's northern Thai Binh province had contracted the disease, increasing fears that it is beginning to spread from person to person. She is the second nurse in a week to have gone down with the flu, which until now has mainly been caught from poultry. Experts have long warned that illness in health workers would be the first sign that the disease had begun to be infectious in humans, bringing a pandemic much closer.

Pandemics occur when a new virus, to which no one is immune, spreads rapidly among people. Experts are unanimous that this will inevitably happen with bird flu, though they are unable to predict when. The World Health Organisation said: "The world is now in the gravest possible danger of a pandemic." Ministers admit that a pandemic would "rapidly" reach Britain thanks to air travel, and could not be prevented from spreading. They have ordered 14.6 million courses of an anti-viral drug, the only defence at present available.

But, as The Independent on Sunday revealed last week, the drugs will take up to two years to arrive. Professor Pennington is worried about the length of time and accused the Government of being "very relaxed" about the possibility of a pandemic. "They hope that by the time they have to spend money the problem will have gone away," he said. "It is rather reminiscent of BSE."

The Government expects one in every four people in the country to catch the flu, if the pandemic breaks out. But it officially predicts the death toll at "around 50,000" and stresses that this would be no more than the four times the normal annual flu death rate.

This appears to be based on two highly optimistic assumptions. First, it assumed that it will kill only 0.37 per cent of those it infects, an estimate based on normal flu. Second, the figure is based on just one wave of the flu.