Air passengers are at risk of developing deep vein thrombosis even on flights lasting just a few hours, a new study suggests.

Scientists who monitored the health of passengers flying between Stansted in Essex and airports in Italy found that a small proportion suffered DVT within two to three hours of take-off. The findings challenge the assumption that only long-haul passengers suffer blood clots, which can lead to collapse and death.

Campaigners hoping to raise awareness of DVT have claimed for some time that short flights pose a risk to passengers, and they say the risk extends to those travelling in business and first class where there is more leg room. Interim results of the latest research, involving 568 passengers, will lend weight to their campaign.

Professor Gianni Belcaro, of G d'Annunzio University in Italy, who has conducted a number of travel-related DVT studies, led a team of experts who examined passengers flying between the UK and Italy from May to September this year. The passengers, aged between 25 and 65, were screened by ultrasound for blood clots both before and after their flight.

The researchers discovered clots in 4.3 per cent of the high-risk subjects after the flight, with two passengers going on to develop pulmonary embolisms possibly related to their trip. Those with an increased risk of DVT are women on the Pill and HRT, people who have recently had surgery and pregnant women. Alcohol also increases the risk.

Professor Belcaro said: "The results show passengers are at risk of developing blood clots even on short flights. In fact, our research suggests most blood clots develop in the first two to three hours of a journey and grow larger and more dangerous with time.

"The problem can be worsened if travellers are then transferring straight to a car or coach for a long journey, or if they wait hours in airports."

Professor Belcaro recommended that people making any kind of journey where they were seated in cramped conditions should take some simple precautions.

These include taking plenty of exercise during the trip, drinking water and wearing flight socks to help the blood circulate and reduce the likelihood of clots developing.

Professor Belcaro said it was unusual to publish interim results when a study was continuing. The decision had been taken, he said, because the findings were considered to have great significance.