The Institute of Cancer Research experts analysed the entire three billion letter sequence of DNA-coding in identical twins to reveal what sets off the disease.
They hope the findings, published in PNAS journal, could lead to new drugs to fight the condition at source.
Leukaemia is the most common cancer diagnosed in children.
It affects a third of young cancer sufferers and kills 100 children a year in the UK.
The twins studied by the researchers had the most common form of leukaemia that affects children – acute lymphoblastic leukaemia (ALL) which is a cancer of the white blood cells.
It is already known that multiple faulty genes are linked to the condition and that environmental factors probably act as triggers along the way. But the precise sequence of events leading up to a diagnosis of ALL is unclear.