People who take long-haul flights are nearly trebling their chances of suffering deep vein thrombosis (DVT), according to a new study.
Dutch scientists assessed the risk of air travel by surveying 9,000 staff who worked for international companies over a period of more than four years.
They found 52 went on to have thromboses after flying - 22 of which were within eight weeks of a long-haul trip.
The authors of the study, published in the journal PLoS Medicine, calculate there was a 3.2 in 1,000 chance of having a thrombosis in those "exposed" to long-haul travel, compared with a one in 1,000 chance for those who had not flown.
The risk was particularly high in employees under 30, in women on the contraceptive pill, and people who were particularly short, tall or overweight.
They concluded: "The results of our study do not justify the use of prophylaxis such as anti-coagulant therapy for all long-haul air travellers, since this may do more harm than good.
"However, for some subgroups of people with a highly increased risk, the risk-benefit ratio may favour the use of prophylactic measures."
A Department of Health spokeswoman said it will look carefully at the report and amend its advice to travellers if necessary.
She said: "Department of Health advice is to move your feet around, or get up and walk around as regularly as you can.
"It is also important to avoid dehydration by having regular non-alcoholic drinks."