The H5N1 bird flu virus could change into a form able to spread rapidly between humans, scientists have warned.
Researchers have identified five genetic changes that could allow the virus to start a deadly pandemic.
Writing in the journal Science , they say it would be theoretically possible for these changes to occur in nature.
A US agency has tried unsuccessfully to ban publication of parts of the research fearing it could be used by terrorists to create a bioweapon.
According to Prof Ron Fouchier from the Erasmus Medical Centre in the Netherlands, who led the research, publication of the work in full will give the wider scientific community the best possible chance to combat future flu pandemics.
"We hope to learn which viruses can cause pandemics and by knowing that we might be able to prevent them by enforcing strict eradication programmes," he told BBC News.
He added that his work might also speed the development of vaccines and anti-viral drugs against a lethal form of bird flu that could spread rapidly among people.
The H5N1 virus has been responsible for the deaths of tens of millions of birds and has led to hundreds of millions more being slaughtered to stop its spread.
The virus is also deadly to humans but can only be transmitted by close contact with infected birds.
Coughs and sneezes
It is for this reason that relatively few people have died of bird flu. Latest World Health Organization (WHO) figures indicate 332 people have died of the illness since 2003.
Health officials are concerned though that the H5N1 virus could one day mutate into a form that could be spread between humans through coughs and sneezes through the air.
This could, they fear, result in a lethal pandemic that could spread rapidly across the world killing tens of millions of people.
It is only now that a study has confirmed that the emergence of such a deadly virus is theoretically possible.
A group led by Prof Fouchier wanted to find out which genetic changes were required to enable the H5N1 virus to mutate into a form that could be transmitted from person to person through the air.
His team compared the genetic structure of the bird flu virus with those responsible for earlier human flu pandemics.
The researchers found five key differences, which they reasoned could be the mutations required for airborne transmission of the virus.
They confirmed their theory was correct by genetically engineering those changes into the H5N1 virus which they found could then be spread between ferrets through coughing and sneezing.
A team from Cambridge University then looked to see whether such a mutation could emerge naturally and if so its likelihood.
The researchers studied the genetic structure of 3,000 bird viruses and 400 that occur in humans.
They found some of these viruses had two of the key changes needed to become airborne. Mathematical modelling suggested it was indeed possible for a virus to develop the three further changes required during the course of an epidemic.
It is the first time it has been shown that it is possible for bird flu to become airborne, but the research team was unable to determine precisely how likely this was to happen.
Prof Derek Smith, who led the analysis, said more information was needed.
He said researchers required a better understanding of how flu viruses were transmitted between people in order to develop a clearer idea of the likelihood of the emergence of an airborne strain of bird flu.
"These are difficult things to find out," Prof Smith told BBC News.
"What this work enables us to do is to prioritise particular experiments to obtain this information".
It is clear though that the emergence of an airborne mutation of H5N1 is unlikely. Were it not it would have emerged already.
But researchers want to be able to calculate the risk of such a virus emerging more precisely in order to help public health officials in their contingency planning.
News of Prof Fouchier's work, and another similar study by Yoshihiro Kawaoka published this May in the journal Nature, prompted the US National Security Advisory Board for Biotechnology (NSABB) to ask both journals last November to redact some sensitive parts of the research.
The NSABB believed the information could be used by terrorists to create a bioweapon.
The scientists who carried out the research, and the journals concerned, considered suggestions as to how the results could be redacted in the journals, but distributed to bona fide researchers who urgently needed the information.
But they concluded such a system was unworkable.
"You can't share information with so many people in the field and keep it confidential," according to Prof Fouchier.
Editor in chief of the journal Science, Dr Bruce Alberts, said the publication of the research in both Science and Nature had "shone a spotlight" on the need to deal more effectively with research that could be misused by terrorists - so called "dual use research of concern" (Durc).
"It has become clear that we will need to work toward the establishment of a comprehensive, international system for assessing Durc, one that includes transparent procedures to allow selected access to any information omitted from a scientific publication to those with a need to know".
But Prof Fouchier questioned whether a system of asking scientific journals to censor Durc work is ever workable or even appropriate.
"The general mode should be that science should be freely available so that the wider scientific community can build on the research," he said.
"I have a hard time identifying research papers that you shouldn't publish. So I'm not sure whether we should ever go down this alley".
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