The influenza vanished in 1968, meaning anyone born after that time would have limited or no immunity, according to the WHO.

“If the virus gets loose, it can easily cause an influenza epidemic,” the WHO’s Klaus Stohr told the BBC.

“If this virus were to infect one person, it would spread very rapidly.”

The samples, of the strain known as H2N2, were accidentally included in testing kits sent out in recent months by the College of American Pathologists.

The company, after a request by the US government, on April 8 wrote to all the laboratories asking them to destroy the samples.

A second message sent on April 12 reiterated the request and provide confirmation that all the H2N2 samples were destroyed, “and that any case of respiratory disease among laboratory workers be investigated and notified top national authorities,” said the WHO.

There have as yet been no reports of infection amongst lab workers, said the organisation.

No details on why the 1957 pandemic strain, which killed between one and four million people, was included in the test kits that are routinely sent to labs.

The H2N2 strain has not been included in the flu vaccine since 1968.

Dr Nancy Cox, head of the influenza branch at the US federal Centres for Disease Control and Prevention in Atlanta, said officials strongly doubt it was deliberately planted or that it was an act of bioterrorism.

“It wouldn’t be a smart way to start a pandemic to send it to laboratories because we have people well-trained in biocontainment,” she said.

Test kits are used by labs for internal quality control checks to demonstrate it is able to correctly identify viruses, or to get certification by the College of American Pathologists.

Kits include blind samples. Usually the flu virus included in the kits is a strain that is currently circulating or has done so recently.