Not being able to move around may be what increases the risk

They looked at nearly 2,500 patients who had been treated for a deep vein thrombosis or blood clot in the lungs.

In the three months before the clot, some 12% reported a "minor leg injury". Just 4% of a clot-free control group had hurt their leg in the same period.

Writing in the Archives of Internal Medicine, the team speculated that loss of activity could cause the problem.

Injury damage to blood vessel walls in the legs may also increase the risk of clotting in that area.

"The association appeared to be local because injuries in the leg were associated strongly with thrombosis, while injuries in other localtions were not associated with thrombosis," wrote lead author Karlijn van Stralen from Leiden University Medical Centre.

Deep vein thrombosis is a rare condition, affecting around one or two mainly older people in every thousand.

It becomes a serious condition when the clot in the leg vein breaks off and travels through the body to the lung, where it becomes lodged again causing severe breathing difficulties.

Untreated, up to one in ten people who suffer a pulmonary embolism die as a result.

Low risk

The link was strongest when the injury had occurred in the month before the clot, and was also stronger in individuals with genetic or other predisposing factors.

The team suggested their study meant GPs need to be aware of these risk when treating someone for a minor leg injury.

Dr David Keeling, expert in DVT at the Oxford Haemophilia Centre and Thrombosis Unit, said the findings seemed credible and corresponded to his experience of treating patients.

"We now know of a number of factors which may increase your risk by threefold - flying, and the contraceptive pill, for instance.