Airlines have known about the possible links between long-haul flights and deep vein thrombosis (DVT) for 10 years, a report has claimed.
The report by BBC's Panorama programme also said dozens of airlines had turned down or ignored requests from scientists to study blood clots among passengers.
One recent study has suggested that as many as one in 10 long-haul passengers could be at risk from DVT, commonly called "economy class syndrome".
DVT is a condition in which a small, potentially fatal blood clot forms, often in the deep veins of the legs.
It becomes potentially deadly when a part of the clot breaks off and blocks a blood vessel in the lungs.
The problem came to public notice following high-profile cases like that of 28-year-old Emma Christofferson, from south Wales, who died after a 20-hour flight from Australia.
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