But the national eye-test programme for pre-school children was stopped in 1997.

This means some children could be being denied the chance of a cure, the researchers warn.

It could also mean they are denied careers later in life, such as being a pilot, which require good sight in both eyes.

Children with the common eye disorder amblyopia, commonly known as "lazy eye", have one eye which does not see properly despite wearing glasses.

The RNIB has always campaigned for all children to be screened for all eye problems at an early age

RNIB spokesman

Researchers from the Children of the 90s study, based at Bristol University, say bringing back testing for children under three could mean better sight for those with the condition.

The Royal National Institute for the Blind (RNIB) said it had always backed early sight tests for children.

Patch

For the study, 2,000 children were either offered repeated eye tests from the age of eight months, or were not tested until they were three.

All those found to have amblyopia were sent to their local hospital for treatment, which usually involves wearing a patch over the good eye to encourage the muscles in the "lazy eye" to develop.

By the time the children in the study were seven-and-a-half, those whose eye problem had been detected before they were three were four times more likely to be cured compared with those who had been diagnosed later.

The pre-school sight test programme was abandoned in 1997 because the NHS Centre for Reviews and Dissemination found there was no evidence that early treatment worked better than treatment at school age.

Whether the state should offer vision testing for pre-school children has been hotly debated

Dr Cathy Williams

But Dr Cathy Williams, who led the Children of the 90s project (Avon Longitudinal Study of Parents and Children), told BBC News Online that was because the research had not been done, not because there was proof testing did not work.

There are now recommendations for testing to be offered to children aged four and five.

At the moment, the situation is patchy, with some local education authorities offering screening for children at primary, or even nursery schools.

However in some areas there is no programme, and in other places it varies from school to school.

Better treatment

Dr Williams said: "Whether the state should offer vision testing for pre-school children has been hotly debated since the 1997 report.

"We hope our findings will stimulate further investigation as to whether testing before three could be offered, which could well lead to better sight for children who need treatment for amblyopia."

"Our findings would be an argument for offering screening for pre-school children, but further research is needed into whether that's practical or feasible."

An RNIB spokesman said: "The RNIB has always campaigned for all children to be screened for all eye problems at an early age.

"If there are no local screening programmes available, parents can have their children's eyes tested for free on the NHS.

"It's the case with amblyopia that the younger it's detected, the better.

"At the very least, children should be screened as they enter primary school."

The research is published in the British Medical Journal.