One of the women has not yet been told of the unnecessary operation and hospital managers said there may be other cases of misdiagnosis.
The errors happened at the Kent and Canterbury Hospital, which was at the centre of a cervical screening scandal in 1995 in which at least eight women died and 90,000 cervical smears had to be rechecked.
The errors were revealed in a review of all cancer diagnosis services at the hospital. It exposed one "rogue" histopathologist who has been ordered to have "remedial training".
The hospital is now checking all the breast biopsies - samples of tissue removed for diagnosis - he examined between 1990 and 1997. Hundreds of women are involved and the hospital admitted it is likely that more were given inappropriate treatment. However, the review by the Royal College of Pathologists concluded that the overall error rate was within acceptable standards.
Experts from the college looked at 2,621 patient histories and identified seven cases where "diagnostic discrepancies" had "a definite influence on clinical outcome". A further 35 cases were defined as "diagnostic misrepresentation" or "oversights" which could have affected the way people were treated. They said some mistakes were inevitable in a service that relied on human judgement but the consultant identified for retraining had made at least three times more mistakes than his colleagues.
David Astley, chief executive of the East Kent NHS Trust, said: "So far it is, sadly, clear that two patients had unnecessary mastectomies. We have seen one of them and told her and her family what has happened. The other lady is abroad." The review of the pathologist's work would involve "a few hundred women" and take "months rather than years" to complete.
The Kent and Canterbury Hospital laboratory in Canterbury has now been closed.Reuse content