The "internal wiring" of breast cancer can predict which women are more likely to survive or relapse, say researchers.
The study shows that breast cancer is 11 separate diseases that each has a different risk of coming back.
The hope is that the findings, in the journal Nature, could identify people needing closer monitoring and reassure others at low risk of recurrence.
Cancer Research UK said that the work was "incredibly encouraging" but was not yet ready for widespread use.
The scientists, at the University of Cambridge and Stanford University, looked in incredible detail at nearly 2,000 women's breast cancers.
They went far beyond considering all breast cancers as a single disease and beyond modern medicine's way of classifying the tumours.
Doctors currently classify breast cancers based on whether they respond to the hormone oestrogen or targeted therapies like Herceptin.
The research team analysed the genetic mutations inside the tumour to create a new way of classifying them.
Previous work by the group has shown breast cancer is 11 separate diseases, each with a different cause and needing different treatment.
By following women for 20 years, they are now able to show which types of breast cancer are more likely to come back.
Prof Carlos Caldas tod the BBC: "This is really biology-driven, it's the molecular wiring of your tumour.
"Once and for all we need to stop talking about breast cancer as one disease, it's a constellation of 11 diseases.
"This is a very significant step to more precision-type medicine."
It showed that triple negative breast cancers - one of the hardest types to treat - were not all one class of cancer, but two.
Prof Caldas said: "One where if women have not relapsed by five years they are probably cured, but a second subgroup are still at significant risk of later relapse."
The research could help inform women of their future risk, but may also change the way their cancers are treated.
There were four subgroups of breast cancer that were both driven by oestrogen and had a "markedly increased" risk of recurrence.
These women may benefit from a longer course of hormone therapy drugs like tamoxifen.
Cancer Research UK estimated that 12,300 women a year have such types of cancer in the UK.
By James Gallagher, health and science correspondent, BBC News
Cancers tend to be named after the place you find them: breast, colon, prostate, lung - the list goes on.
But it has long been known that this is not good enough.
This study shows the future of personalised medicine and tailoring treatment to the specific causes of someone's cancer.
Studies are already under way looking at which treatments may work best for different subtypes of breast cancer.
And other research groups are trying to get a similar insight into other forms of cancer too.
However, the way the scientists analysed and sorted the cancers is still too complicated to be introduced to the NHS.
It will need refining into a form that could be used as a routine way of analysing a woman's cancer.
Much larger studies involving up to 12,000 women are also planned, so that researchers can be certain of their results.
Prof Caldas said: "I would not recommend it clinically yet, but we really are committed to making this available.
"We are totally committed to having an NHS test, we haven't patented any of this."
Prof Karen Vousden, Cancer Research UK's chief scientist, said: "We're still a way off being able to offer this type of detailed molecular testing to all women and we need more research to understand how we can tailor treatments to a patient's individual tumour biology.
"But this is incredibly encouraging progress."
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