Britain would be "overwhelmed" if a strain of the deadly bird flu virus spread to the UK, a leading scientist warned today.

Professor Neil Ferguson said it would be impossible to stop the spread of the virus once it reached British shores.

He warned that the only chance of avoiding a global disaster is to eliminate the strain of the disease as soon as possible at its source in south-east Asia.

"What can we do if it hits our shores? We couldn't stop it. There would be a constant number of new cases and we would be overwhelmed very rapidly," said Prof Ferguson.

The H5N1 avian flu strain, which infects poultry, has already killed more than 50 people in Vietnam, Cambodia and Thailand.

Experts say almost all the human bird flu deaths so far have resulted from an animal passing the virus to a human, but the World Health Organisation has consistently warned that if the virus mutated it could spread rapidly from person to person, creating a global pandemic.

The virus could claim even more lives than the 1918 Spanish flu pandemic, when between 20 and 40 million people died worldwide.

Scientists predict that if a pandemic strain emerged and nothing was done, it would cross international boundaries in just two to three months.

In just one year, half the world's population - more than 3 billion people - would be infected.

Prof Ferguson reached his conclusion after his team of scientists investigated the worst-case scenario for bird flu. A separate team also looked at the possible spread of the disease.

Both teams focused on Thailand, one of the places at highest risk from bird flu, and both teams came to similar conclusions. Their computer simulations revealed that even if the strain became transmissible between humans it could be halted in time - but only with swift, coordinated international action.

The new deadly version of the virus would have to be identified while confined to only about 30 people, Prof Ferguson's team found.

Antiviral drugs would have to be distributed rapidly to the 20,000 individuals nearest those infected, and the outbreak limited to fewer than 200 cases.

Prof Ferguson warned that failure to take action swiftly enough would result in catastrophe. "If the virus got as far as Britain, it would effectively be too late," he said.

The scientists are now conducting urgent research to discover to what extent deaths could be prevented in the UK.

Prof Ferguson's group found that an international stockpile of 3m courses of antiviral treatment would be enough to contain an outbreak.

Last month, the government revealed that Britain is set to stockpile 2m doses of the bird flu vaccine for medical and emergency staff.

Government officials said the vaccine against H5N1 could be a "first line of defence" for priority groups.

The other research team, led by Dr Ira Longini, from Emory University in Atlanta, Georgia also concluded that stockpiling a vaccine could contain the virus.

"Our findings indicate that we have reason to be somewhat hopeful. If - or, more likely, when - an outbreak occurs in humans, there is a chance of containing it and preventing a pandemic," said Dr Longini.

"However, it will require a serious effort, with major planning and coordination, and there is no guarantee of success."

Currently the antiviral drug Tamiflu, made by Swiss-based pharmaceutical company Roche, stands the best chance of curbing pandemic bird flu. It is effective against multiple strains of influenza and can also be used as a preventative treatment.

Prof Ferguson said he understood Roche was prepared to make free donations of Tamiflu to the World Health Organisation to help meet the threat.

A spokeswoman for Roche said: "We are in discussions with the WHO but details are still being finalised."

The two research papers were published in the latest issues of the journals Nature and Science.