Dr Samantha Knight, and a team from the John Radcliffe Hospital in Oxford, examined the chromosome ends of 284 children with unexplained moderate to severe learning disabilities.

They also looked at 182 children with unexplained mild learning disabilities - and used a control group of 75 men without learning disabilities.

Chromosomes are the microscopic strings of DNA in cells which carry each individual's genetic information - in order.

Parents also had abnormality

The researchers found that the chromosomes of 7.4% of children with moderate to severe problems, and 0.5% with mild disability had slightly jumbled DNA at their ends.

Parents of the children with re-arranged chromosome ends were then also tested. Ten of the children with jumbled chromosome endings had a parent with a DNA abnormality.

In nine of these families, all other relatives with learning disabilities also showed similar abnormalities.

The results, which were published in medical journal The Lancet, have been greeted with caution by learning disability groups, who warn that genetic research is only useful in certain contexts.

The causes of learning disabilities are unknown in up to 70% of cases

Researchers wrote in The Lancet: "Our data support the view that the rearrangements are the cause of children's disabilities. First, the prevalence of small chromosomal abnormalities in the group enrolled through the genetics clinics was significantly larger than that in the control group."

But they added that many other factors had to be considered.

A spokesman for Mencap said: "The process of identifying explanations for disability, including learning disability is on-going. The JR work seems to have the potential to provide some further explanations, though it would be necessary to replicate the work elsewhere before starting to draw any conclusions.

"The most useful research is research which provides clues to treatment of conditions which will be worse without treatment.

"Research which simply indicates the possibility that a child might have some degree of learning disability provides a very insecure basis for difficult ethical and personal decisions about having children or continuing with a pregnancy."