The World Health Organization (WHO) has declared a global flu pandemic following an emergency meeting in Geneva, according to reports.
The new level reflects the fact that the swine flu (H1NI) virus is spreading in at least two regions of the world.
So far cases are being seen on the rise in the UK, Australia, Japan and Chile.
The decision to move it to the next level is not an indication that the virus is causing more deaths, however.
The swine flu virus first emerged in Mexico in April 2009.
It has since spread to 74 countries. Official reports say there have been 28,000 cases globally and 141 deaths with figures set to increase.
The declaration of a pandemic will trigger drug makers to speed up production of a swine flu vaccine and prompt governments to devote more money to containing the virus.
While international health officials have said the flu appears to be less deadly than the annual bouts of seasonal flu that sweep the globe each year, they have warned that the virus could mutate into a more lethal strain during the Southern Hemisphere's coming winter flu season.
There are also worries that poorer countries with inadequate health facilities could be overwhelmed with cases they do not have the capacity to treat.
There have been more than 800 cases in the UK with some areas of Scotland being particularly hard hit.
The British Government has been stockpiling antivirals such as Tamiflu and has ordered vaccine, some doses of which could be available by October 2009 when it is expected the virus could become more virulent due to the winter weather.
The UK's chief medical officer, Sir Liam Donaldson, said the WHO declaration of a pandemic would not significantly change the way the UK was dealing with swine flu at the moment, which involves investigating each individual case.
But he told the BBC that there could be some minor changes as to who received antivirals.
"The difference is that the Health Protection Agency has learnt a lot about approaching this question of antiviral prophylaxis and they are going to be treating the closer contacts of the cases, rather than the more far-flung contacts, because they feel that that is supported by what they know so far about how the disease is transmitting," Donaldson said.
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