Anita Froggatt agreed to undergo the mastectomy at the Chesterfield Royal Hospital after analysis of a lump removed from her right breast apparently showed it was malignant. Two weeks later it emerged that the slide with her healthy breast tissue had been confused with the slide of another woman, who had cancer.
Yesterday the hospital said it took full responsibility for the error and that it was investigating how it had happened. A member of the laboratory staff has been suspended on full pay.
Christopher Day, medical director, said: "We acted immediately and saw the patient as soon as this error came to light. It is a tragic situation and we take full responsibility. We have given her a frank explanation of events and, of course, a full apology."
Mrs Froggatt, 28, who has a 10-year-old son, told The Mirror that she was still too shocked by the news of the unnecessary operation to be angry. She said: "I can never forgive them for this. They have robbed me of my womanhood. It's unthinkable what they've done to me, and it's all for nothing."
She added: "It's unbelievable how one lab technician's work can sail through as gospel without being checked and checked again before such drastic surgery. It beggars belief. You trust the NHS. You believe what you're told."
She has lost her factory job since the operation, which included removal of the lymph nodes in her armpit, because movement of her arm is now restricted.
The hospital said that it would not contest a claim for compensation, which some estimates have put as high as pounds 500,000.
Phil Bowen, a solicitor who is representing Mrs Froggatt, said: "In many years of dealing with medical negligence claims, this is undoubtedly the most appalling case that I have experienced.
"The distress which has been caused to Anita is unimaginable. To even begin to recover from this traumatic experience my client will require counselling and reconstructive surgery." He added: "Both Anita and myself would wish to make it clear that this is an unusual mistake and neither of us would wish this to affect the use of the breast screening service by any woman who discovers a lump in her breast."
In the past, patients have had limbs wrongly amputated but greater precautions are now taken and these errors have been eliminated. In the early 1990s, healthy patients in Birmingham were wrongly diagnosed with bone cancer and underwent chemotherapy as a result of errors by a pathologist in reading tissue samples. But breast cancer charities said yesterday that they had not heard of slides being mixed up in a laboratory before. A spokeswoman for Breast Cancer Care said: "It is absolutely horrifying. With today's health care and the screening available, nothing should go wrong."
The hospital said that the other woman in the mix-up had received the correct treatment because her slide was not misread.