Doctors can predict how severe a child's leukaemia will be by looking for three faulty gene patterns.
Test searches for genetic clues
Those with the poorest prognosis could be treated with more aggressive therapy, UK experts told a cancer conference in Birmingham.
The Southampton University team focused on genes linked to the most common form of childhood leukaemia, called ALL.
The test, which took six years to develop, is already beginning to be used as standard.
About 450 children in the UK have ALL (acute lymphoblastic leukaemia ).
The aim of treatment is to destroy the leukaemia cells and allow the bone marrow to work normally again.
Chemotherapy can get rid of the leukaemia cells in about three-quarters of people with ALL, putting them into disease remission.
But some will need more intensive treatment, which may include more chemotherapy and possibly bone marrow transplants.
The test developed by Dr Anthony Moorman and his colleagues helps doctors spot and treat the 25% of children with ALL who need more intensive treatment.
It looks for "gene fusions" - the accidental joining of DNA of two genes.
The genes involved include etv6 joined with runx1, bcr joined with abl and a gene called mill that can fuse with several different partners.
By looking at more than 1,000 children with ALL they found which patterns of gene fusion predicted poorer outcomes.
In particular, those with fusion of etv6 with runx 1 - about 20% of children with ALL - had better outcomes than others.
Those with multiple copies of runx 1 had some of the poorest outcomes.
Dr Moorman said that carrying out these gene tests in addition to normal tests for ALL improved the accuracy of prognosis.
"It's an important confirmatory test to the traditional method of cytogenetics, which looks at chromosome numbers and structure.
"As a result of our work, doctors now use these three gene fusion tests as standard and will treat their patient accordingly."