Up to one in 10 older airline passengers on long distance flights may be at risk of potentially fatal blood clots, dubbed economy class syndrome, according to a clinical trial.

Researchers from University Hospital, London, report a "surprisingly large" proportion suffering from "symptomless deep vein thrombosis" in their calves, a condition which could lead to lung clots.

They also suggest that the threat may be reduced by wearing below-the-knee elastic compression stockings, since none of the people wearing them in the trial exhibited any sign of thrombosis.

The study, involving 200 volunteers over the age of 50 travelling on flights of eight hours or more, divided the passengers into two groups. Half of them wore the stockings.

They were checked when they left Britain and after their return. None of the stocking wearing passengers had symptomless DVT in their calves compared with one in 10 of those who did not wear stockings.