The findings could open up new pathways in preventing and treating the debilitating disease.

However, the number of cases has been gradually increasing since the 1950s. Its cause is unknown.

But scientists have uncovered evidence that clusters of cases tend to be found in relatively small geographical areas over a short period of time.

They also noticed that children born in winter tend to be at higher risk.

This suggests that the condition is likely to be linked to an environmental factor that poses a varying level of risk at different times of the year - a bacterial infection would seem to fit the bill most closely.

Confounding the odds

Scientists from Cancer Research UK's Paediatric and Familial Cancer Group at the University of Manchester analysed 1,045 cases of the disease, all from the north west of England, dating from 1954 to 1998.

They found that in certain years the number of cases of the cancer found in children that lived closed together was much higher than would be expected by pure chance.

Team leader Professor Jillian Birch said: "Our results indicate that environmental factors are involved in causing brain tumours in children and the most likely explanation for the pattern we have seen is that one or more types of infections are responsible."

Scientists now believe that further research is needed to confirm that infections are involved and to try to identify the agents.

Significant step

Professor Gordon McVie, joint director general of Cancer Research UK, said: "It is vital that we learn more about the causes of childhood cancers and this is an important step in the right direction.

"We'd thought infection might play a role in the development of children's brain cancer, but up until now we had no evidence to support the theory."

Sir Paul Nurse, also joint director general of Cancer Research UK, added: "We believe that infections play a role in a number of cancers, so it is interesting that a virus or bacterium may also be implicated in the development of brain tumours in young people.

"If an infection is playing a role, this might lead to new ideas for preventing and treating this important disease."

The research is published in British Journal of Cancer.