and Egypt the authorities are worried about the spread of the virus in the dog and cat population.

In Britain and the US, public health studies suggest that a full-blown pandemic of bird flu or something like the respiratory disease SARS could knock out a critical mass of the working population. Some sixty per cent of the nursing and medical services could be out of action within 10 days, according to a study at the Defence Academy of the UK Staff College. This would mean the armed services would have to be called in to help.

The threat to public order - a scenario out of the Day of the Triffids or the Quatermass Experiment - is what really alarms Downing Street. An American study has suggested that in a worst case, half the population would go down with bird flu – roughly the scale of Europe's Black Death of 1348. Half of those would have to go to hospital and millions would die in the first wave.

The government has laid in stocks of 14.6m courses of Tamiflu, one of two known medicines capable of combating H5N1. However, pharmaceutical competitors have

claimed that Tamiflu would only be effective for a very short time, and the WHO says the virus appeared to be resistant to Tamiflu in at least two known cases.

A glimmer of hope has been raised by research in America and the UK that suggests that there is something about humans that means the bird flu virus would have to mutate twice in order to 'unpick the lock' in the glycans, or sugar chains that line human airways and lungs.

But for Gordon Brown the problem is to warn and prepare, without causing public panic. Security experts are pretty sure a new version of the Black Death is odds on to happen. Their biggest concern now is that terrorists could use the viruses as a new weapon of mass destruction. Leaders like Brown know they have been warned.