Experiments found when the chemical PRH (Proline-Rich Homeodomain) was reduced, tumours divided and progressed more easily.
Tests on tumour cells and mice suggest the protein could switch off genes that fuel the disease.
The breakthrough could also be applicable to other types of cancer as the activities of PRH are not confined to the breast.
Dr Padma Jayaraman, of Birmingham University, said: “We made the significant finding high levels of PRH actually blocked the formation of the tumours.
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“Therefore our data suggests PRH can block tumour formation in some breast cancers.”
Her researchers say the protein could hold the secret to suppressing the growth of breast cancer tumours.
Dr Jayaraman said: “It’s possible drugs could be developed that target PRH - or the things it controls.”
It could also help better determine the prognosis for patients, according to the findings published in Oncogenesis.
Dr Jayaraman, of the university’s Institute of Cancer and Genomic Sciences, said: “PRH is a protein that controls and regulates when genes are switched on or off.
“However - prior to our research - the role of this protein in breast cancer has been poorly understood.
“Public databases show, in a large number of breast cancer patients with a poor prognosis, the activity of the PRH gene had decreased.
“However, it was not known whether the amount of PRH protein was also lower in these patients as protein levels had not been recorded.”
Her researchers used a special staining process on breast cancer tissue removed during biopsy to observe the levels and location of PRH proteins in diseased cells.
They identified changes in PRH in the tumour cells compared to normal cells that were consistent with the decreased activity of the PRH gene in the public database.
Dr Jayaraman said: “In the laboratory we found when PRH protein levels are reduced in a breast tumour the cells are more able to divide - speeding up the progression of the tumour.
“Moreover, we identified some of the genes which are regulated by PRH and specifically contribute to the increased cell division.”
The researchers also carried out tests in a tumour model of mammary cancers -increasing PRH levels to observe the effect.
Dr Jayaraman said: “This was a clincher - when we confirmed our original findings in mouse mammary cells.
“We propose monitoring PRH protein levels or activity in patients with breast cancer could be particularly important for assessing their prognosis.
“In addition, since PRH is known to be important in multiple cell types, this work has important implications for other types of cancer.
“We are now working to investigate the importance of PRH in prostate cancer and in cancer of the bile duct, a type of liver cancer.”
Breast cancer is the most common form of the disease among women in the UK with over 50,000 diagnosed each year in the UK.
One-in-eight will develop breast cancer in their lifetime - with about 11,400 dying from it every year.