Aviation insurers could avoid paying compensation for deep vein thrombosis (DVT) claims because of a get-out clause in the Warsaw Convention.
The High Court last month gave the go-ahead for 300 claimants to launch an unprecedented group legal action against airlines.
But a leading aviation insurance lawyer said the court must first determine whether DVT could be considered an accident under the Warsaw Convention, which applies to all commercial international flights.
The Convention allows the recovery of compensation only in respect of personal injury or death caused by an accident.
Tim Brymer, an aviation lawyer with Cameron McKenna, said: "Unless the claimant can demonstrate an unusual event during the course of the flight, the claim would fail."
British Airways and Virgin are among 30 airlines facing claims for damages. BA welcomed the group action, saying it streamlined the legal process, but said it would resist any claims.
One industry analyst claimed payouts could be as high as £500,000 per person and £100,000 for bereaved families.
The High Court ruling granted permission for action by relatives of 50 passengers who died after suffering DVT, and up to a further 250 who survived.
Aviation insurers are remaining tight-lipped about the potential effects of the claims on their balance sheets.
It is feared that up to one in ten passengers suffers from DVT. Scientific studies say it is not necessarily limited to cramped, economy class seats. Estimates claim that up to 2,000 people per year are killed by it.