Computer gamers as young as 12 have suffered potentially-deadly deep vein thrombosis (DVT) after becoming so absorbed in their games they hardly moved for more than three hours, a new study has revealed.

The cases have alarmed medical experts who say its virtually unknown for children so young to suffer DVT.

DVT, where blood clots form from lack of movement in the legs, is normally restricted to older patients who have thicker blood, are less active and are more likely to be overweight.

It is more commonly associated with long-haul air travel but studies led by the Medical Research Institute of New Zealand has shown increasing incidence from people’s sedentary lifestyles due to computer working, TV and film viewing and gaming.

The 12-year-old had played for four hours with his legs in a kneeling position when he suffered a blood clot. Like other cases including a 15-year-old, he was young, healthy and with no other known risk factors.