Businesses and travel companies were facing the prospect of multimillion-pound losses yesterday following a surprise announcement by the European Union's health chief urging travellers to avoid non-essential trips to the USA and Mexico.

The news came on a turbulent day for the international stock markets which saw shares in major airline and travel companies plunge, while those in drug companies soared.

Speaking at a summit in Brussels, the European Commissioner for Health, Androulla Vassiliou, urged people "to avoid non-essential travel" to areas affected by the global flu pandemic. "They should avoid travelling to Mexico or the United States unless it's very urgent for them," she said. The advice – in stark contrast to that issued by the Foreign Commonwealth Office and World Health Organisation – was particularly unnerving for the transatlantic business travel sector which, since 2007, must abide by the Corporate Manslaughter Act, legislation that was introduced in 2007 to bring companies to justice over the death of employees.

"Companies have to be mindful of any international travel because firms are all too aware that if one of its employees is taken ill during a business trip, it can be held liable," said Martin Ferguson, a journalist at Travel Trade Gazette.

Nigel Cooper, the managing director of Motivcom, an international business events group, added: "Statements like this can be cataclysmic. I am not underestimating the potential for a medical disaster, but such judgements should be left to health professionals."

The warning came on a day when shares in international airlines and travel companies tumbled. British Airways fell at one stage more than 9 per cent, while Thomas Cook and TUI Travel were down 5 per cent and 3 per cent respectively.

North America Travel Service, one UK tour operator specialising only in travel to the US, called for things to be "kept in context".

"Although this directive refers to travel to the USA, we have to remember that the US is two-and-a-half times the size of Europe and the mild cases reported so far are in just five of the 50 states," said Karen Farrar, the firm's marketing manager.