One of the largest studies of deep vein thrombosis (DVT) in air travellers involving almost 900 long-haul passengers has found that one in 100 suffered "clinically significant" symptoms of the condition.

The result is nine times higher than the rate expected in the population and is the clearest indication yet of the frequency of DVT in air-travellers, which has been a source of anxiety among passengers for years.

Although the condition is commonly known as economy-class syndrome, the researchers said business-class travellers were also affected and duration of travel was more important than the quality of the seating. Professor Richard Beasley, of the Medical Research Institute of New Zealand, who led the study published in The Lancet, said: "This is probably as close as we will get to working out the true incidence. The term economy-class syndrome is now redundant with a better term being air-travellers' thrombosis."

The finding is far below the 10 per cent incidence suggested in an earlier study in The Lancet in 2001. That was based on sensitive tests for the presence of small symptomless blood clots. The new study considered only clinically significant blood clots, in the legs or the lungs, with the potential to cause pain or breathlessness.

The researchers studied 878 frequent long-haul air travellers who spent an average of just under 40 hours in the air within six weeks, the rough equivalent of a flight from New Zealand to Britain and back.

They were given blood tests before and after flying and those with signs of DVT or pulmonary embolism were investigated with ultrasound and pulmonary angiography, an X-ray examination of the blood flow to the lungs.

The researchers found nine travellers with signs of blood clots, five in the legs (DVT) and four in the lungs (pulmonary embolism). Up to 10 per cent of cases of pulmonary embolism were life threatening, Professor Beasley said. He added that the passengers were an "average group of adult travellers" with a low to medium risk of developing DVT. He said: "I would not have expected more than one event [blood clot] in this population."

The link between prolonged sitting in cramped conditions and deep vein thrombosis was first spotted among survivors of the Blitz in the Second World War who slept in deck chairs in air-raid shelters. The problem stopped when the deckchairs, which pressed on the back of the legs, were removed.

DVT has been linked with travel by car and rail, but reports of cases have been less frequent, probably because of more frequent breaks in which passengers can get up.

Professor Beasley described a case of pulmonary embolism in a 32-year-old office worker who sat at a computer many hours a day. In a paper in the European Respiratory Journal this year he called the case an example of "ethrombosis, a 21st-century variant of venous thromboembolism associated with immobility".