Airlines could be held liable for hundreds of deep vein thrombosis (DVT) claims if two legal actions are successful.

Watford legal firm Collins is preparing to test airlines' liability for the potentially fatal blood clotting condition which is sometimes developed by passengers after long-haul flights. Last week travel insurer Club Direct introduced automatic legal expenses cover to enable DVT sufferers to sue airlines. It claims that more than 2,000 people die each year from DVT.

But Andrew Innes, an aviation underwriter at Lloyd's insurer Wren, was sceptical as to whether the test cases launched by Collins would lead to large payouts for insurers.

He said: “Airlines have been receiving claims for DVT-related injuries for many years, but very few have lead to litigation as airlines have preferred to settle claims out of court with relatively small payouts.”

Law firm Collins has acted for claimants in a number of transport-related cases, including the Hatfield train crash.

Lawyers for the firm have written to British Airways and JMC, owned by Caledonian Airways and Thomas Cook, asking them to disclose internal documents detailing what they know about the problem of DVT.

One case is being brought by a widow whose 40-year-old husband died 24 hours after getting off a trans-atlantic flight. The other test case involves a woman university student.

Des Collins, senior partner of the firm, said: “We have formally contacted British Airways and JMC over the two cases and asked them to respond within 21 days. If we do not receive satisfactory replies we will go to the high court and request that the airlines disclose all their documentation.”

He said the action was needed to break down what he alleged was the airlines' “wall of silence” over the issue.

Collins added that the current levels of compensation offered to DVT sufferers under the airlines' Warsaw international convention was poor.

A spokesman for British Airways said it had received a pre-claim notice and was investigating the matter.

JMC issued a statement which said: “JMC Airlines can confirm that it has received correspondence from solicitors representing a passenger who travelled to the Caribbean last December.

“The personal injury claim is relating to DVT. The company is investigating the allegations.”