Thomas Lamb, 68, from Cardiff, was another victim

She collapsed at Heathrow after a 20 hour long flight home.

Emma's parents have become staunch campaigners for more research into the causes of DVT - originally dubbed "economy class syndrome" because of the number of cases affecting passengers sitting in cheaper seats on aircraft and in cramped conditions.

John and Ruth Christoffersen have demanded a full investigation into the death of their daughter Emma, who died last October.

Research released last week showed that one in 10 people travelling on long-haul flights could develop blood clots.

Scientists revealed that 40 times more people suffered DVT after long-haul flights than previously thought.

But many of the clots are "symptomless" and do not go on to develop into larger and potentially fatal clots.

Stockings

The new research - published in the medical journal The Lancet - raised concern among passengers.

It has also provoked controversy with some scientists, who dispute the high number of cases found by consultant surgeon John Scurr and his team.

Mr Scurr, who said the study was a preliminary one, told the BBC he recommended passengers on long flights should wear compression stockings.

"It's not an economy class problem, it's not related to the back of the aircraft, it can affect anybody if they have the right risk factors," he told BBC Radio 4's Today programme.

Law suit

"Immobility predisposes people who are probably already at risk for other reasons," he said, adding that very long bus or train journeys could present the same problem.

Relatives of 14 UK DVT victims have launched a multi-million pound law suit against the airlines concerned.

Roger Wiltshire, Secretary General of the British Air Transport Association told Today "We accept there is a risk with immobility.

"That is why the airlines have done an awful lot in the past 12 months to improve the advice they give to passengers about exercise and sensible hydration in flight."