(Corrects day to Wednesday throughout)
* Scientists halted research voluntarily a year ago
* US raised concerns virus could be used by terrorists
* Benefits of studies outweigh risks, scientists say
By Kate Kelland
LONDON, Jan 23 (Reuters) - Scientists around the world declared an end on Wednesday to a moratorium on research into mutant forms of the deadly H5N1 bird flu that can be transmitted directly among mammals and had raised international biosecurity concerns.
Announcing their decision to resume what they say are risky but essential studies of the avian flu strain, the scientists said the work would only be carried out in the most secure sites in countries that agree it can go ahead.
That will allow work to start again in key laboratories in the Netherlands and elsewhere but not yet in the United States or U.S.-funded research centres, pending further regulatory moves there.
Scientists voluntarily halted work on the transmission of H5N1 a year ago due to fears that scientific details about how to create such a potentially dangerous virus could be used for bioterrorism.
Flu experts said they had recognised those fears and worked hard to calm them, and now it was time to push on. They say the studies are essential for a deeper understanding of H5N1, which many fear could one day spark a lethal pandemic in humans.
The research may also boost efforts to develop global flu "biosurveillance", early warning systems, as well as drugs and vaccines to protect against the threat.
"We fully acknowledge that this research - as with any work on infectious agents - is not without risks," the scientists wrote in a letter published jointly on Wednesday in the journals Nature and Science. "However, because the risk exists in nature that an H5N1 virus capable of transmission in mammals may emerge, the benefits of this work outweigh the risks."
The letter was signed by 40 flu researchers from the United States, China, Japan, Britain, the Netherlands, Canada, Hong Kong, Italy and Germany.
"The lifting of the moratorium will undoubtedly lead to more scientific revelations that will have direct consequence for human and animal health," said Wendy Barclay, a flu virologist at Imperial College London and one of the letter's signatories.
All research into H5N1 transmission was halted in January 2012 after labs at the University of Wisconsin in the United States and at the Dutch Erasmus Medical Centre in Rotterdam created mutant forms that can be transmitted directly among mammals, meaning they could in theory also pass between people.
Currently, bird flu can be transmitted from birds to birds, and birds to humans, but not from humans to humans. When it does pass from birds to humans, it is usually fatal. Scientists are concerned the same mutations needed to make it transmissible among mammals in a lab could one day happen in nature.
News of the work, which emerged late in 2011, prompted the U.S. National Science Advisory Board for Biosecurity to call for the scientific papers about it to be censored to prevent details falling into the wrong hands.
The censorship call sparked a fierce debate about how far scientists should be allowed to go in manipulating dangerous infectious agents in the name of research.
Barclay said this had been a "knee-jerk response from certain quarters previously naive of this approach, expressing horror that scientists were brewing up deadly diseases."
During the moratorium, the World Health Organisation recommended that scientists should explain the biological and other security measures they use to contain the virus and make more effort to show why the research is so important.
"The laboratories have expanded on their containment and security system ... and I think the value of the results has been recognised. Therefore, the ... recommendations were satisfied," John McCauley, director of the WHO collaborating centre for flu research at Britain's National Institute for Medical Research said in a statement on Wednesday.
In their letter, the scientists said the aims of the moratorium had been met in some countries and are close to being met in others, so it was now time to "declare an end" to it.
"H5N1 viruses continue to evolve in nature," they wrote. "Because H5N1 virus transmission studies are essential for pandemic preparedness and understanding the adaptation of influenza viruses to mammals, researchers.... have a public health responsibility to resume this important work." (Reporting by Kate Kelland; Editing by Robin Pomeroy)