Many of the children affected were born prematurely

They say this is due to more low-birthweight babies and those with severe neurological conditions, who are more prone to blindness, surviving.

Deprivation and being from an ethnic minority also increase a child's risk, they said.

The scientists said more efforts should be put into treating the conditions which cause blindness.

Severe visual impairment and blindness in childhood in the UK is more common ... than previously assumed Dr Jugnoo Rahi, Institute of Child Health,

They contacted ophthalmologists and paediatricians across the UK to find out how many children were newly diagnosed as being severely visually impaired or blind in 2000.

They received data for 439 children - though they say this is likely to be an underestimate as some were probably missed.

Previous studies, which looked at small groups of sight-impaired children, suggested the total number of children who were blind should have been around 300.

They said their findings showed that by the age of 16, six per 10,000 children would have developed extreme sight problems or blindness.

The majority were affected during their first year of life.

Three quarters of those studied had other diseases or disabilities in addition to their sight problems.

Complex conditions

The researchers said the increase in the numbers of babies born prematurely and surviving explained many of the cases.

Because the babies are not fully developed, they are at a higher risk of developing a range of conditions linked to sight problems than full-term babies.

Cases of retinopathy of prematurity, where oxygen treatment given to premature babies can cause blindness, was also a factor.

The increased survival rates for children with complex neurological conditions such as cerebral palsy, brain tumours and other childhood cancers also contributed to the figures, they said.

Children from ethnic minorities are also at a higher risk.

The team from the Institute of Child Health said much more work needed to be done to look at preventing or treating the conditions that led to child blindness.

They found that around three-quarters of the newly diagnosed children had blindness that could not currently have been prevented or treated.

The World Health Organization has made preventing visual impairment and blindness in childhood an international priority.

But many countries do not record information about incidence and causes to enable them to assess what needs to be done to prevent and treat visual impairment problems.

Interventions

Dr Jugnoo Rahi from the Institute of Child Health who led the research, told BBC News Online: "The focus of research should be on childhood diseases in general.

"For instance, research into cerebral palsy is bound to have an impact on child blindness.

"The fruits of this labour will have an effect worldwide, since the trends in childhood visual disability in the UK can, inevitably, be expected to be replicated in countries currently in economic transition."