World airlines, facing lawsuits over blood clots suffered by passengers on long-haul flights - the so-called economy class syndrome - said there was no conclusive link between flying and the disorder.

An Australian law firm has launched suits against three international airlines, KLM of the Netherlands, Australia's Qantas Airways and British Airways, as well as Australian air safety body CASA.

Compensation specialists Slater & Gordon said they had lodged three test cases in the Victoria Supreme Court over deep vein thrombosis (DVT).

The cases, not expected to be heard by the court for 18 months at the earliest, seek unspecified damages.

British Airways, Europe's largest airline, and KLM both said there was no definitive proof linking blood clots to air travel.

Australian Naomi Forsyth, 21, who is one of the three suing for compensation, said she wanted passengers to be aware of the troubles they faced.

She added that swelling in her leg from the blood clot would affect her for the rest of her life.

In the wake of worldwide publicity after the death last October of British woman Emma Christofferson, many airlines began to show passengers videos on suitable in-flight exercises and to print DVT warning pamphlets.

"For thousands of people this information came too late for them to take simple precautions or assess their health suitability for a long flight," said Slater & Gordon lawyer Paul Henderson.

Slater & Gordon had been contacted by 2,792 potential DVT claimants since the death of Christofferson, the firm said. It said 197 of the cases involved deaths.