Deep-vein thrombosis is the formation of a blood clot in a deep vein. It commonly affects the leg veins, such as the femoral vein or the popliteal vein or the deep veins of the pelvis.
Researchers in Southampton and New Zealand questioned 200 patients who were admitted to hospital for either blood clots or heart problems and compared how long each group had remained sitting, both in total and in one period without getting up.
"The risk of developing blood clots with prolonged seated immobility is largely unrecognized. However, this study has shown that it is at least as important a factor as long-distance air travel," the Telegraph quoted Prof Richard Beasley, from Wellington Hospital, New Zealand, as saying.
"It is similar to the situation with the risk of blood clots with long-distance air travel - it was not until there was research into its role that the real extent of the problem was appreciated.
"This study provides preliminary evidence that prolonged seated immobility at work may represent a risk factor for venous thromboembolism [deep vein blood clot] requiring hospital admission.
"Both the maximum time seated at work during a 24-hour period and the maximum time seated without getting up were associated with an increased risk of VTE," he added.
Each additional hour spent sitting without getting up increased the likelihood of blood clots by 20 per cent, the study found.
All workers who commonly sit at their computer for most of the day should do the same leg and foot exercises, such as flexing the ankles that are recommended during long-haul flights, it was advised.
They should also take regular breaks away from their desk to walk around, said the researchers.
The authors, whose study is published in the Journal of the Royal Society of Medicine, said the increased risk associated with sitting may be because of increased pressure in the veins in the legs.
Prof Beasley said: "These factors may be exacerbated by sitting in cramped conditions, or if intense prolonged concentration results in reduced muscle activity."