The bird flu virus H5N1 has mutated into a form that makes it more infectious to humans, increasing the risk of a human pandemic, researchers have found.

The changes, which only affect the virus circulating in Europe and Africa, are worrying although they have not yet transformed it into a pandemic strain, Yoshihiro Kawaoka, of the University of Wisconsin-Madison, who led the research, said.

The outbreak of avian influenza caused by the lethal virus H5N1 began in Asia in 2003 and spread around the world, leading to the slaughter of hundreds of millions of birds.

More than 30 countries have reported outbreaks in the last year, mostly in wild birds such as swans. The virus has infected 329 people since 2003, of whom 201 have died, according to the World Health Organisation.

Governments have responded to the threat by stockpiling millions of doses of the anti-flu drug, Tamiflu, and preparing emergency plans for dealing with a pandemic.

In the UK, the disease broke out on a Bernard Matthews turkey farm in Suffolk and more than 100,000 poultry were culled after the virus was apparently imported from Hungary. But there have been no human cases in the UK or Europe.

Writing in the Public Library of Science journal, PLoS Pathogens, Dr Kawaoka and colleagues say recent samples of virus taken from birds in Europe and Africa all carry the mutation, which makes the virus more likely to grow in the nose and throat of humans.

"I don't like to scare the public, because they cannot do very much," Dr Kawaoka tolds the Reuters news agency. "But at the same time it is important to the scientific community to understand what is happening."

Humans have a lower body temperature than birds and the avian virus finds it difficult to grow in the cooler conditions of the human throat. The mutated virus is better adapted to these cooler conditions, making transmission more likely.

"The viruses circulating in Europe and Africa all have this mutation," Dr Kawaoka said. "So they are the ones that are closer to human-like flu."

All flu viruses are evolving constantly and other mutations were necessary to convert H5N1 avian flu into a human pandemic strain, he said.

European specialists are divided about the likelihood of a human pandemic caused by the H5N1 virus. Sceptics say that as the virus has been circulating for four years, and if it were going to mutate into a pandemic strain it would have done so by now.

Those of a gloomier cast of mind say the risk is still high that it could evolve into a lethal strain that would kill on the scale of the 1914 pandemic when an estimated 40 million people died.

Professor Angus Nicoll, the flu co-ordinator at the European Centre for Disease Prevention and Control in Stockholm, said: "Agnostics like me are in the middle. My view is that we have to prepare for a pandemic even though we haven't seen any changes in the behaviour of the virus in the field."

Scientists are agreed that, even if the H5N1 strain does not cause a human pandemic, it is likely that a future strain will do so. There were three pandemics in the last century – in 1918, 1957 and 1968 – and more are expected.

Professor Nicoll said countries were having to decide whether to buy newly developed vaccines against H5N1. Some such as Finland and Switzerland, had placed large orders, but that involved gambling on a human pandemic caused by the H5N1 strain happening in the next few years. Other countries, including the UK, had ordered smaller amounts for certain groups in the population. "It is a difficult choice," Professor Nicoll said.