The world was told not to panic yesterday after swine flu was finally declared an official pandemic by the World Health Organisation.

However the World Heath Organisation chief Dr Margaret Chan said the move does not mean that the swine flu virus is causing more severe illness or more cases of death.

The announcement makes swine flu the first pandemic for 40 years, with the virus spreading to 74 countries since it first struck in Mexico in April 2009.

Official records suggest there have been 30,000 cases worldwide and 141 deaths, with figures increasing daily. The mortality rate is similar to that of regular flu and most victims suffer from only a mild form of illness.

The last flu pandemic was in 1968 and was named the Hong Kong virus, killing around 1,000,000 people.

Flu experts say the biggest danger from swine flu would be if the H1N1 virus mutated into an even deadlier variation, but Dr Chan said it appeared 'pretty stable' and was under 'constant and careful' watch by qualified agencies around the globe.

Never in history has a previous pandemic been detected this early or watched so closely right at the initial point of exposure, she said, but countries must maintain a thorough and 'continuous vigilance'.

Experts have warned that poorer nations face the greatest risk.

One factor which prompted the move to the current level six pandemic - the highest - was that in the southern hemisphere the virus seems to be crowding out normal seasonal influenza as winter begins leading to masses of cases.

The World Health Organisation announcement came as another 25 cases of swine flu were confirmed in England and 26 in Scotland, taking the UK total to a staggering 848.

Sir Liam Donaldson, the Chief Medical Officer of the United Kingdom, said the country was one of the best prepared countries in the world to deal with the virus.

He said the current UK government policy of containing outbreaks would continue, although advisers are considering whether to restrict the use of Tamiflu to high risk cases which would include victims and close contacts of victims.