A third of air passengers who suffer a potentially fatal blood clot are aged under 40. The most comprehensive study yet into British victims of 'economy class syndrome' challenges the widely held belief that the elderly are most at risk from deep vein thrombosis, or DVT.

An analysis of 544 British passengers who developed a clot during the past three years found that 170 were in their teens, 20s or 30s.

It had been assumed that those aged 50 to 70 are most vulnerable because they have poorer circulation. But the survey, carried out by the Aviation Health Institute, found that of those who died a quarter were under 40. The youngest was Sara Brown from Swansea who died 18 days after returning from Australia. She was 23.

The findings confirm that women are almost twice as likely to die, especially those on the pill or undergoing hormone replacement.

Incidents were spread equally between economy and business class, and experts believe that the number of victims will continue to rise as the cost of flying falls.

A petition containing 17,000 signatures was handed in at Downing Street last week demanding a change in the law to cover air travel. 'What we really need is a corporate manslaughter charge against the airlines,' said Farrol Kahn, director of the Aviation Health Institute.