Contact lens wearers are at the mercy of a bug that is found in tap water and gnaws through the eyeballs causing blindness, scientists have warned.

With the Acanthamoeba parasite also found in dust, in the sea and in showers and swimming pools, millions of people are at risk worldwide, including Britain's 3.7million contact lens users.

The actual number of infections is small but treatment is long, painful and not completely effective, meaning some Britons are left blind every year.