It means the swine flu virus is spreading in at least two regions of the world with rising cases being seen in the UK, Australia, Japan and Chile.
The move does not necessarily mean the virus is causing more severe illness or more deaths.
Flu expert Professor John Oxford, said people should not panic as the outbreak was milder than others seen in the past century.
"It is global and fulfilling the requirements of a pandemic but I don't think anyone should worry because nothing drastic has happened between yesterday and today."