It does not mean the virus has become more deadly, just that its geographical spread is now global.

The WHO warned that countries already hit by the virus should prepare for a “second wave” of cases.

The number of infections has climbed in the United States, Europe, Australia, South America and elsewhere.

It is the first global pandemic for 41 years.

The virus first emerged in Mexico in April and has now spread to 74 countries with a reported 28,774 cases and 144 deaths, according to WHO’s latest tally of laboratory-confirmed infections.

Twenty-five new cases have been confirmed in the UK, bringing the total number to 822. There have been no deaths.

A Department of Health spokesperson said: “The localised cases of swine flu found in the UK have so far been generally mild in most people, but are proving to be severe in a small minority of cases.

“We are continuing to work to slow the spread of the disease and to put in place arrangements to ensure that the UK is well-placed to deal with this new infection.”

In a statement to member countries, the WHO said it decided to raise the pandemic alert level from phase five to six.

The decision was made after the UN health agency held an emergency meeting in Geneva, Switzerland, on swine flu with its experts.

Moving to phase six will trigger a large scale production of vaccines and prompt governments to devote more money towards efforts to contain the virus.

It will also raise questions about why the step was delayed for weeks as the virus continued to spread.

The WHO has urged countries not to close borders or restrict travel and trade.

It said: “At this early stage, the pandemic can be characterised globally as being moderate in severity. We remain in close dialogue with influenza vaccine manufacturers.”

According to the WHO’s own pandemic criteria, a global outbreak means a new flu virus is spreading in at least two world regions.

Sky’s health correspondent Thomas Moore said any move to declare a pandemic did not mean the virus itself was growing in potency.

He explained: “This is a marker if you like of geographical spread. It’s not an indication that the virus is becoming more severe.”

GlaxoSmithKline is already working with a key ingredient of the swine flu vaccine to see how quickly doses can be produced.

And other major pharmaceuticals like Sanofi Pasteur have also been working on a vaccine after WHO gave them a “seed stock” of the virus last month.

However, drug giants say it could take up to six months before large amounts of a swine flu vaccine are available.