As fears of a swine flu pandemic worsen and the World Health Organisation (WHO) moves its alert system to five - one level away from a full pandemic - shares in airline and travel companies have taken a hit.

The WHO has reported that there have been confirmed cases of swine flu - a modified form of H1N1 influenza, which can spread from to human to human, as opposed to avian flu (H5N1), which seems to be largely limited to birds - in Mexico, where it originated, the US, Austria, Canada, Germany, Israel, New Zealand, Spain and the UK.

The UK Foreign Office is advising against all but essential travel to Mexico and the Mexican secretariat of health has closed museums and other public venues, such as bars and nightclubs. For example, Six Flags, the US-based theme park firm, has announced that it has closed its Mexican parks to conform with advice from Mexican authorities.

Airlines around the world have seen shares fall, including British Airways (-8 per cent), Cathay Pacific (-8 per cent), Qantas (-4 per cent), Lufthansa (-9 per cent), Air France (-7 per cent) and Continental Airlines (-16 per cent).

Travel operators Thomson, Thomas Cook and Airtours have all announced that they would stop trips to Mexico, and are offering various flight deferments or refund deals.

Also, according to the BBC, Russia, China, Ukraine and Thailand have banned imports of pork from North and Latin America, despite the fact that the flu does not actually affect pigs. Details: www.fco.gov.uk