Contact lens cases of the future may carry a warning: "This product could cause blindness."

A study carried out at Moorfields eye hospital in central London found that soft contact lens users, particularly those using extended wear lenses, had far more risk of developing a bacterial eye infection than those wearing hard lenses.

The infection, known as microbial keratitis, an inflammation of the cornea, can lead to blindness if that part of the eye is perforated or scarred.

The study, published in the Lancet, found users of extended wear contacts most at risk 21 times more likely to develop the infection than hard lens wearers.

Ordinarily soft lenses had a 3.6 times risk, and polymethylmethacrylate hard lenses a risk 1.3 times that of regular hard lenses.

Soft lenses and extended wear lenses are increasingly prescribed these days because they are easy to fit.

The British College of Optometrists is so concerned about these results that it had advised opticians that extended wear lenses should only be prescribed when they are the only medical option.

Any practitioner failing to warn his patients of the risk would be acting unethically, said the college.