Their study, published in the journal Nature, predicts that swine flu has "full pandemic potential," and is likely to reach full global proportions within the next six to nine months.

The study, based on Mexico’s experience of the outbreak, did not estimate the likely death toll from such a pandemic. But it did conclude that the H1N1 virus is as dangerous as the virus behind the 1957 flu pandemic which killed 2m people worldwide.

According to the World Health Organisation, the number of laboratory-confirmed swine flu cases has reached 5,251 in 30 countries around the world, with 61 deaths.

The Spanish flu pandemic of 1018 caused an estimated 50m deaths worldwide.