According to a new study, several CT scans on children increases the risk of developing brain cancer or leukaemia by three times.

A team of researchers lead by the Newcastle University looked into the NHS medical records of almost 180,000 young patients and found that these scans increased the risks by three times in children who undergo multiple scans.

On the other hand, they wrote that the benefits of the scans usually overweighed the negative aspects. They said that the study shows that the scans should only be used when necessary and that the ways to reduce their radiation must be developed.

During a CT scan, a -ray tube rotates around the patient's body for creating a digital detailed images of the organs and other parts for the doctors to study. During the longest stud of this kind in the world, the researchers studied records of patients under the age of 21 who underwent CT scans at various hospitals in the UK between 1985 and 2002.

Dr Mark Pearce, an epidemiologist from Newcastle University who led the study, said: "We found significant increases in the risk of leukaemia and brain tumours, following CT in childhood and young adulthood. The immediate benefits of CT outweigh the risks in many settings."

He added, "Doses have come down dramatically over time - but we need to do more to reduce them. This should be a priority for the clinical community and manufacturers."

The findings were published in the journal The Lancet.