A year ago, most air passengers were led to believe that the greatest danger to their health came from a drink-fuelled, mid-flight fracas. Times have changed, and now the biggest threat to travellers is thought to be potentially lethal blood clots striking down unsuspecting victims.

The condition, known as deep vein thrombosis (DVT), is reported to have affected numerous people around the world, with some dying after leaving a flight.

Airlines accept there is a link between long periods of immobility and DVT, but refuse to acknowledge that the conditions in an aircraft cabin could heighten the risk.

Heightened publicity in recent weeks has led to a perceptible shift in attitude, however, with a number of the world's leading international carriers, including British Airways, Singapore Airlines and Japan Airlines, issuing more explicit guidelines to passengers on how to avoid developing the condition.

This was followed in late January with a commitment from the UK government to carry out urgent research into DVT. "Airlines must accept that they have the principal role to play, both in minimising risk and providing effective information to their passengers before they fly," says Chris Mullin, Britain's transport minister.

Yet airlines continue to refuse to accept a direct link with flying, despite mounting medical evidence. The view of Dr Michael Davies, the specialist adviser to the UK House of Lords (upper, unelected house of parliament) sub-committee that compiled a report on air travel and health last year, is typical of the industry.

"My own view is that the jury is still out," says Dr Davies, who was director of health services at British Airways until 1996.

The lack of a direct link is crucial to airlines, which are facing a growing number of law-suits from passengers who suffered DVT and feel they should have been warned of any risks.

A lawyer who represents carriers in personal injury cases points out that previous attempts by passengers to sue for DVT-related illnesses have failed because airlines were able to show that the flight exacerbated a pre-existing condition. Airlines' condition of carriage, established under international charter, exempt them from liability in such cases.

All that could soon change with the publication of groundbreaking research that will claim to establish a link between flying and DVT for the first time.

The research, conducted over eight months by John Scurr, a consultant vascular surgeon at London's Middlesex Hospital, studied 200 people aged over 50 before and after they flew long distances. Mr Scurr says the study found a "significant" sample developed small blood clots after the flight, although none showed symptoms of DVT.

"Undoubtedly the study will make a link between long-haul flying and DVT," he says. "But what we need is more studies to establish who is at risk." Mr Scurr suspects a comprehensive study will reveal that the risks are limited to a small group of passengers.

The findings contradict research published by a Dutch team last October, which suggested there was no proven link.

Mr Scurr is aiming to use his research as a launchpad for a global study which would have the added benefit of making more people aware that the problem does exist. "If we produce a study in England all the English will believe it and if we do one in the US then all Americans will believe it. But what we want to avoid is a 'not-from-here syndrome'," says Mr Scurr.

He was due to present his evidence to a group of 12 leading international experts on DVT at the end of last month in an attempt to persuade them to pool their research.

Patrick Kesteven, a consultant haematologist in north-east England, has also studied the link between DVT and flying and welcomes Mr Scurr's initiative. "Most peopl