A lack of movement during the flight is thought to cause the problems
Researchers from France studied millions of passengers flying into Paris' Charles de Gaulle Airport between 1993 and 2000.
They found that the longer the flight the greater the risk for the passenger, although the risk of developing fatal clots was still relatively low.
Dr Frederic Lapostolle, from the Avicenne Hospital in Bobigny, France told the New England Journal of Medicine that people who flew over 3,100 miles, were 150 times more likely to get a serious blood clot than those who flew on shorter flights.
Clot risk
The phenomenon is dubbed "economy class syndrome" because it is thought to be caused by inactivity and a lack of room on flights.
Studies since 1954 have noticed a link between air travel and clots.
The French team found that although no clots were seen for people travelling less than 1,550 miles, clots did appear in one out of every 2.5m passengers who flew between 3,100 and 4,500 miles.
And the rate increased to one in 376,000 for passengers travelling 4,500 miles to 6,215 miles and one in 210,000 for people flying over 6,215 miles.
Over the seven year study, 56 people were found to develop the serious lung clots.
But there are warnings that the problem could have been ever greater.
Because the study only examined people who were affected within an hour of their plane landing, the risks could in fact be higher.
People who suffered fatal clots while in flight and those who developed symptoms after leaving the airport were not included in the study.
Minimising risks
Researchers say that although moving around during a flight has not been shown to reduce the risk of clots passengers should still do some minor exercise such as walking or leg exercise during their journey.
They suggest avoiding alcohol during the flights, wearing loose clothing and elastic stockings and avoiding sitting with crossed legs.
In an editorial Dr Jack Ansell, of Boston University Medical Center said: "These finding are clearly the tip of the iceberg."
He said that although drugs like aspirin reduce the clot risk that "it may be premature and even dangerous" to recommend anti-clotting drugs to air passengers.