"It represents a five-fold increase," Professor Michel Coleman of the London School of Hygiene and Tropical Medicine told a conference delving into the causes of the increase.

From about 10 cases per million population in England and Wales in 1911-1915, cases rose to about 46 per million at the end of the century.

"The evidence suggests a steady increase in the occurrence of leukemia in this country and in others," said Coleman.

Leukemia is the most common childhood cancer, accounting for nearly one-third of all cases. Most of the rise is in children aged 1 to 4.

Boys have about a 10 percent higher risk of developing the disease, according to Coleman.

Professor Denis Henshaw of the University of Bristol in south-western England and chairman of the conference, said a possible cause could be environmental agents or chemicals that were not around 50 years ago.

Children are thought to be predisposed to the illness at birth by something that occurs in the womb but they do not develop it unless it is triggered by causes as yet unknown.

Ionising radiation, electromagnetic fields, viruses, infections and chemicals and pesticides are thought to be possible triggers.

Professor Alan Preece, also from the University of Bristol, presented research showing the unborn child is particularly sensitive to the effects of exposure to such agents.

In laboratory and animal studies, Preece found levels of such compounds were higher in the fetus than in the placenta or the mother.

"The environmental agents cross the placenta and accumulate in certain foetal organs, varying according to the nature of the agent," Preece said.

"The exact levels are as yet unknown but we know that childhood leukemia is initiated in utero and this could well be a factor in the initiation."

More than 200 doctors and specialists are attending the week-long meeting, organized by Children with Leukemia, Britain's leading charity devoted to conquering the illness.