The prospect of a summer of travel chaos has been raised a notch after several airlines, led by British Airways and Virgin Atlantic, have started refusing to check-in holidaymakers suspected of suffering from swine-flu like symptoms, unless they can provide a “fit to fly” note from their doctor.

A number of people have already been denied access to flights after appearing ill as they arrived at the airport check-in.

The new measures are also being applied to the seas, with P&O Cruises amongst the companies now vetting passengers, who are now required to fill in a medical questionnaire before boarding. Those suspected to be suffering are then given a medical examination by staff prior to boarding.

A BA spokesman confirmed that a “very small number of cases” where people displaying symptoms had been advised not to travel.

He said: “Our staff are trained on what to look out for if someone has swine flu or any other communicable disease. The staff seek medical advice and anyone with swine flu would be advised they are unfit to travel. Even if they disagree with us we have the power to stop them going on board.”

Howeve the move has received criticism among the medical profession, saying doctors would not be able to identify everyone carrying the virus and that the process would be a drain on health workers’ time.

Peter Holden, the British Medical Association’s lead representative on pandemic flu, told The Times that he was advising doctors not to issue the notes because they would not be worth the paper they were written on.

“It is a total and utter waste of time,” Dr Holden said. “A fit-note is only going to be valid at the moment of issue. You could easily become ill between leaving the GP’s surgery and reaching the airport. It flies in the face of government efforts to relieve pressure on doctors, and we have much more important work to do than this.”

Meanwhile eight British schoolchildren on a trip to China remained in hospital with swine flu, and over 50 of their classmates and teachers quarantined.

The Association of British Insurers (ABI) said that travelling against official advice would often void any policy and insurers would not pay out in the event of illness while abroad.

He said he expected insurers to be “flexible” when it came to dealing with claims relating to swine flu, especially as often the sufferer is not officially diagnosed.

He said: “There is an issue of diagnosis. Under normal circumstances travel insurers would require confirmation from a GP but in these circumstances it is difficult as sufferers are being told not to visit their GP. We expect our members to be flexible on the issue.”

A report released today by Ernst & Young says that if the number of new cases each day rises to 10,000 by August, and last six months, this would lead to a 7.5 percent fall in GDP this year.

The warning comes as the World Health Organisation confirmed fears that the H1N1 virus is moving with “unprecedented speed” as modern travel carries the virus, spreading further in six weeks than earlier pandemics in six months.