Scientists have created a life-threatening virus that closely resembles the 1918 Spanish flu strain that killed an estimated 50m people in an experiment labelled as "crazy" by opponents.

US researchers said the experiments were crucial for understanding the public health risk posed by viruses currently circulating in wild birds, but critics condemned the studies as dangerous and called on funders to stop the work.

Scientists at the University of Wisconsin-Madison used a technique called reverse genetics to build the virus from fragments of wild bird flu strains. They then mutated the virus to make it airborne to spread more easily from one animal to another.

"The work they are doing is absolutely crazy. The whole thing is exceedingly dangerous," said Lord May, the former president of the Royal Society and one time chief science adviser to the UK government. "Yes, there is a danger, but it's not arising form the viruses out there in the animals, it's arising from the labs of grossly ambitious people."