A team from the Institute of Cancer Research investigated the causes of acute lymphoblastic leukaemia (ALL) in two to five-year-old children.

The BBC's Daniel Sandord: "The seeds of leukaemia are sown before the child is born"

They found that a vital gene defect implicated in the development of the cancer could be traced back to before birth.

The scientists, led by Professor Mel Greaves, from the Leukaemia Research Fund Centre for Cell and Molecular Biology, also discovered that the gene defect was not inherited from the parents.

ALL is diagnosed in around 400 children in the UK each year.

It accounts for about 80% of leukaemia cases in children, and is most often found among two to five-year-olds.

The discovery of the gene defect is a significant breakthrough, but scientists believe that the mutation alone is not sufficient to stimulate the development of full blown leukaemia.

They are now searching for a second trigger that occurs sometime after a child is born.

Professor Greaves said one possibility was that leukaemia developed as an abnormal response by the body to an infection

Highly sensitive probes

Using highly sensitive probes, the scientists were able to analyse blood samples from new born babies.

They found that they contained a mutated version of a gene called TEL-AML 1, one of the most common molecular abnormalities in children with leukaemia.

It appears that the mutation occurs during the development of the blood cells in the foetus.

Professor Greaves said: "We conclude that the common form of childhood leukaemia frequently originates pre-natally and probably does so in most cases.

"The puzzle of childhood leukaemia is unfolding. There is now a real prospect that it will be solved over the next few years."

Dr Peter Rigby, chief executive of the Institute of Cancer Research, said: "This research marks an important step in the fight against leukaemia.

"We are making real progress and this new knowledge takes us further towards a complete understanding of a disease which affects around one child in 2,000 in the UK."

A major study into childhood cancer, being carried out by the Co-ordinating Committee on Cancer Research, may shed light on the mysterious second trigger for childhood leukaemia.