The gene plays a role in many cases of breast cancer
Scientists have pinpointed a gene linked to more than half of all breast cancers.
The gene, NRG1 (neuregulin-1), is also thought to play a role in many bowel, prostate, ovarian and bladder tumours.
The University of Cambridge team said the breakthrough should provide "vital information" about how cancer spreads.
Experts agreed the finding, published in the journal Oncogene, could represent a very significant advance in the fight against cancer.
The Cambridge team showed that the gene - which helps to suppress the growth of cancer - is located on chromosome 8.
Cancerous cells are known often to miss part of that chromosome, and when the researchers analysed breast cancer samples they found that at least part of the key gene had often been lost.
Everybody is born with an intact NRG1 but it seems that in some cases it can become damaged - leaving the way open for cancer to thrive.
Lead researcher Dr Paul Edwards said: "I believe NRG1 could be the most important tumour suppresser gene discovery in the last 20 years as it gives us vital information about a new mechanism that causes breast cancer.
"We have got strong evidence that the gene is implicated in breast cancer but we have no reason to think it's not the same for other cancers, including prostate and colon cancer.
"Finding out what genes have been turned off in these cancers is enormous help in understanding what has gone wrong with their biology."
Arlene Wilkie, of the Breast Cancer Campaign, which part-funded the study, said: "Knowing the identity of this gene will lead to far more detailed studies of how it works and how it is involved in breast cancer development.
"This research is a major step forward in understanding the genetics of cancer and could open up a host of new strategies to improve diagnosis and treatment."
Lesley Walker, of the charity Cancer Research UK, which also funded the study, said more research was now needed to understand how the gene was silenced, and how exactly it influences the development of cancer.
She said: "It might then be possible to develop ways to bypass the gene or target treatments to the defect."