“The fear is that if you create something this deadly and it goes into a global pandemic, the mortality and cost to the world could be massive,” a senior scientific adviser to the US Government told The Independent, speaking on condition of anonymity.

“The worst-case scenario here is worse than anything you can imagine.”

For the first time the researchers have been able to mutate the H5N1 strain of avian influenza so that it can be transmitted easily through the air in coughs and sneezes. Until now, it was thought that H5N1 bird flu could only be transmitted between humans via very close physical contact.

Dutch scientists carried out the controversial research to discover how easy it was to genetically mutate H5N1 into a highly infectious “airborne” strain of human flu.