They are urging anyone with breathing problems to get in touch with a doctor before stepping on a plane.

Although aircraft cabins are pressurised, it is only to about two-thirds normal atmospheric pressure - the equivalent of standing on a 7,000 foot tall mountain.

And people with lung conditions such as emphysema, severe bronchitis or chronic obstructive pulmonary disease (COPD), who have difficulty getting enough oxygen at sea level, can worsen and dangerously lower the oxygen level in their blood.

Cabin pressures are much lower than at sea level

Consultant chest physician Dr Owen Johnson, a member of the British Thoracic Society, says there is plenty of anecdotal evidence that patients are unwittingly placing themselves at serious risk.

He said: "I have heard of many cases where people have needed oxygen on flights after getting into difficulty.

'Serious danger'

"A small number of patients are putting themselves in serious danger.

"I believe that people simply do not realise the potential problems caused by low cabin pressures."

He said that asthma patients, while at risk if experiencing a severe attack, should normally be able to cope.

The British Thoracic Society is currently developing guidelines for doctors on how respiratory patients should be tested for their fitness to fly.

Blood testing and 50 metre walks

The methods used by doctors vary widely - from simply asking patients to walk 50 metres and seeing if they become breathless, through blood testing of oxygen to placing them on a machine which mimics low atmospheric pressure to test their response.

Each airline has its own procedures to try and spot the passengers most at risk.

Virgin Atlantic has trained cabin door staff to recognise the tell-tale signs and call a doctor.

If contacted in advance, it offers a "special assistance" service to patients, involving extra oxygen supplies.

All airlines carry oxygen cylinders for use in emergencies.

Doctors, many of whom, as passengers, are called upon to help in a crisis, have expressed concern at the lack of lifesaving equipment aboard aircraft.

"Hospital Doctor" newspaper has been running a campaign calling for airlines to spend more on equipment, with the suggestion that heart defibrillators be available on long-haul flights.