Contact lens wearers living in hard water areas of the South East could have a higher chance of catching a rare infection that can cause blindness.

People who use monthly disposable soft lenses are most at risk of picking up the infection Acanthamoeba keratitis (AK), researchers at Moorfields Eye Hospital in London found.

The condition is caused by a microscopic waterborne amoeba which can travel from a contact lens into the eye.

It is thought the organism thrives in the lime scale found in hard water. In severe cases a corneal graft is needed to prevent blindness.

When researchers examined the number of new infections in England and Wales during a two-year period they discovered the largest number of sufferers living in hard water areas, especially in the South East.

Almost 90 per cent of cases were among contact lens wearers.

Dr Cherry Radford is the author of the report, published in the British Journal of Ophthalmology.

She said: "The geographical variation in incidence may be partly related to the increase in risk associated with hard water.

"The fact that water quality can have such an effect on the risk of AK suggests that many contact lens wearers must be letting tap water come into contact with their lenses and storage cases."