Canadian specialist Dr Paul Goss suggests that increasing dietary intake of products containing flax-seed could even prevent the disease.
However, his study involved only 25 women eating his flax seed muffins, and he has called for more research to be carried out to prove his theory.
The women's breast tumours were later removed to see how fast the cancer cells were growing.
The group eating the flax seed muffins had tumours, which were growing significantly less than the others.
This inhibitory effect had been suggested beforehand by studies using animals.
'More definitive proof'
Dr Goss, who presented the results at a Texan cancer conference, said: "It encourages us to believe this is a very significant biological effect in women and we are heading towards more definitive proof that dietary flax seed may prevent breast cancer.
"Our results are exciting because this is the first time anyone has demonstrated these changes in breast cancer with any dietary component."
The different varieties of flax cultivated commercially are used either to make clothing, or as an edible foodstuff when ground down.
Many health claims have been made on behalf of edible flax seed, including protection from heart disease and stroke.
A spokesperson for Imperial Cancer Research Fund said: "The evidence presented here that flaxseed may help treat cancer appears to be encouraging.
Caution sounded
"However, it is difficult to draw any certain conclusions from this study alone. In this case further research with larger studies must be carried out before flaxseed is more widely used."
The power of the chemical ingredients of flax seed, especially if ground up, has led some doctors to warn against patients supplementing their diets with it unless they have a high cancer risk.
Flax seed contains rich concentrations of the plant version of the human hormone oestrogen.
Animals fed large amounts showed abnormalties in prostate glands and ovaries in some experiments, and doctors have warned against taking lots of flax seed during pregnancy.