DVT can be caused by periods of inactivity

The 24 claimants, who also include victims who survived DVT, are trying to prove the airlines are responsible for inducing the condition.

Among them is the family of Emma Christoffersen, 28, from Newport, who died within hours of getting off a flight from Sydney.

A court last year ruled the airlines were not liable but gave the victims leave to appeal. Many of the 55 claimants in that case could not afford to continue the legal process.

They even control the air you breathe but they have no liability for your health John Smith MP

The ruling in December was based on the 1929 Warsaw Convention which says DVT cannot be classed as an "accident", and therefore airlines cannot be held responsible.

'Control'

The 18 airlines involved include British Airways, Qantas, Airtours, International Airways, Monarch Airlines Ltd, JMC Airways Ltd, Virgin Atlantic Airways, and Continental Airlines.

Ruth Christoffersen told the BBC: "They knew there was a link even before Emma was born.

Ruth Christoffersen, is calling for action after her daughter Emma's death from DVT

"They did know, and they chose to do nothing about it."

But the airlines' lawyer Sean Gates, said: "DVT doesn't constitute an accident.

"It is an internal reaction of passengers to the ordinary circumstances of flying, or the ordinary circumstances of sitting on your couch and watching TV."

Vale of Glamorgan MP John Smith, who chairs the DVT awareness group at Westminster, said the inability to sue the airlines was "ridiculous".

"They control every aspect of the environment of the plane when you fly - where you sit, how you sit, what you eat, what you drink. They even control the air you breathe but they have no liability for your health.

"It was alright 75 years ago when they brought in this convention as hardly anybody flew. Today 55 million Britons fly abroad in one year alone and they have no protection whatsoever."

In Australia, a court in Sydney ruled that 59-year-old Brian Povey could take both Qantas and BA to court.

Mr Povey was forced to give up work after suffering an attack of DVT, which he blamed on a three-day return business trip from Sydney to London.

Aviation health bill

Mr Smith is introducing the Aviation Health Bill in Parliament later this week.

It will state that airlines have a duty of care for the "health, welfare and psychological wellbeing of their passengers".

He said it would apply to coach and train travel as well, so the victims of the Potters Bar crash could sue the rail company.

DVT is a blood clot, usually in the leg, which develops when movement is restricted. It can be fatal if the clot reaches the lungs or brain.

The condition was first identified 60 years ago in people sitting in Underground stations for long periods of time as they sheltered from bombing raids.

More than 40 long-haul airline passengers are known to have died from DVT, which is also known as economy-class syndrome.

But scientists remain divided as to whether there is a definite link between flying and DVT.

The hearing was adjourned until Wednesday.