A second wave of the H1N1 virus "would be the biggest of all outbreaks the world has faced in the 21st century," the head of the World Health Organisation (WHO) has warned.
Margaret Chan said in a newspaper interview on Monday the virus could re-emerge in the months ahead "with a vengeance".
"I'm not predicting the pandemic will blow up, but if I miss it and we don't prepare, I fail. I'd rather over-prepare than not prepare," she told the Financial Times.
Portugal on Monday confirmed its first case of the new H1N1 strain, after a woman who had recently travelled back from Mexico tested positive for the virus.
"She was not seriously ill and it is several days now since she displayed any more symptoms," Ana Jorge, the Portuguese health minister, said.
"Her relatives and a group of people who travelled with her [to Mexico] have been tested and none of them have been infected."
Spain's health ministry said on Monday that the number of confirmed cases in the country has risen to 54, from 44 a day earlier, but only 11 victims are in hospital.
Twenty-five people have died from the flu, which has affected 985 people in 20 countries, according to the WHO.
One death was in the United States and the rest in Mexico.
Meanwhile, US health authorities said more than half the nation's states had reported cases of the flu with 226 confirmed infections in 30 states.
Chan's warning came a day after Felipe Calderon, the Mexican president, said his country was "in a position to overcome" the epidemic.
"We have been able to hold or at least reduce the rate of propagation of the virus to contain the epidemic," he said.
Mexican officials are expected to decide on Monday whether to extend a five-day nationwide shutdown of public venues or allow schools and businesses to reopen later this week.
Hospitality sector representatives have said the measure was costing Mexico's economy $100m a day and threatening 450,000 jobs.
Mexican diplomats have also complained to China after dozens of their citizens were placed in quarantine despite showing no signs of having the symptoms of flu.
China's move came after a Mexican woman who arrived on a flight from Mexico was found to be suffering from the virus.
Twenty-four confirmed in Mexico, one in the United States
Countries with confirmed cases: Mexico, the US, Canada, Colombia, Costa Rica, El Salvador, China, South Korea, New Zealand, Israel, Spain, Britain, Germany, Austria, Switzerland, Denmark, France, Italy, Ireland, the Netherlands, Portugal
Countries with suspected cases: Australia, Brazil, Chile, Norway, Guatemala, India
Annual influenza epidemics are thought to result in three to five million cases of severe illness and between 250,000 and 500,000 deaths around the world, WHO says
Ma Zhaoxu, a spokesman for the Chinese foreign ministry, said the isolation was correct procedure and was not an act of discrimination against Mexicans.
Mexican officials said they were preparing to send a specially chartered aircraft to China to pick up any Mexicans who wish to return.
The spread of the H1N1 virus is being closely watched in South America, where a number of countries have confirmed cases and the flu season is about to begin.
The WHO's global alert currently stands at phase five - signalling that a pandemic is imminent - while phase six would mean a pandemic, or global spread, was taking place.
Before raisng the alert to level 6, the highest level, the WHO would need to see the virus spreading within communities in Europe or Asia.
The row has strained what has been a warming relationship between the two countries, with Mexico China's second-largest trade partner in Latin America behind Brazil, and its biggest export market there, according to Chinese statistics.
China was criticised for initially denying the 2003 outbreak of Severe Acute Respiratory Syndrome or Sars, a virus which killed almost 800 people around the world, although with the majority of cases in Hong Kong and China.