All forms of transport involve a small but increased risk of deep vein thrombosis (DVT) on journeys of four or more hours, the study found.

People undertaking long journeys were found by the study to be three times as likely to develop blood clots in the veins of the legs, thighs or pelvis. The clots can be fatal if they travel to the lungs.

DVT has often been described as “economy-class syndrome”, and most of the well-known cases have involved airline passengers. But the study, which involved several research projects, found that the risk was the same whether the journey was made by aircraft, car, train or bus.

The people found to be most at risk from developing DVT were those travelling for more than 12 hours, those using oral contraception and those taller than 1.9m (6ft 3in). The overall risk was found to be small. In all incidences of DVT, only 1 per cent of cases proves to be fatal.

The project was funded by the Department for Transport, the Department of Health and the European Commission, and carried out by a consortium of medical research scientists under the auspices of the World Health Organisation. The Department of Health website offers advice on reducing the risk of developing DVT during journeys. The department advises travellers to move their feet, or walk around as regularly as possible. It is also important to drink water or other non-alcoholic drinks to avoid dehydration.

This month, the House of Lords ruled that travellers who suffered DVT on flights could not sue the airline. The ruling was made against eight families of DVT victims who had sought legal approval to claim compensation from the airline industry.

They argued that factors such as cramped seats played a significant role in causing DVT. Lawyers representing the families have said that they may take their case to the European courts.

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