Authorities in Hong Kong, Malaysia, South Korea and Japan checked airport passengers for signs of illness.
The UK Government ruled out new screening at airports because symptoms can develop two days after contact. Officials here are concentrating on surveillance.
At the weekend the Department of Health sent out a "public health cascade" to all GPs surgeries, primary care trusts, hospital trusts and other health authorities, advising them to look out for suspicious cases. Officials held emergency meetings in London to discuss the health threat.
If the flu arrives here, sufferers will be asked to remain at home, to prevent the spread of infection. A friend or other person will collect stocks of anti-viral drugs for delivery to the sufferer's home.
The NHS has stockpiled more than £500m worth of the drug Tamiflu – enough to cover half the British population – which has proved effective in Mexico.
The Health Protection Agency is leading the strategy from its nerve centre at the Centre for Infectious Disease in Colindale, north London.
"It's being taken very seriously," said Professor Phin. "The WHO have indicated a Public Health Emergency of International Concern – that's the first time it's been done since the inception of the International Health Regulations. This is exceptional and there's a lot of concern."
