Officials were forced to issue the patient with an 'unreserved' apology over the mix-up.
Elizabeth Dawes, a breast cancer nurse from Stafford, was told by staff at New Cross Hospital in Wolverhampton that she had grade 3 invasive breast cancer and needed surgery and chemotherapy.
She underwent extensive surgery as a result of the diagnosis, but four days later the 39-year-old mother-of-one was told her medical notes had been mixed up with those of two other patients.
Ms Dawes called the mix-up 'appalling' and is taking legal action.
She is still taking painkillers almost two years after the surgery and was so traumatised by it she gave up her job at New Cross.
Bosses at the hospital have met with Ms Dawes to offer an 'unreserved apology'.
A spokesman for the Royal Wolverhampton NHS Trust said: "The medical director and nurse director met with Ms Dawes to offer her an unreserved apology for this terrible error and inform her of the trust's investigation into the incident.
"The findings of the investigation were shared with Ms Dawes and a further apology offered. The incident is now part of an ongoing legal claim with which the trust is cooperating fully.
"The trust can confirm that no other patient received inappropriate treatment as a result of this incident."
Ms Dawes has since been prescribed anti-depressants.
Doctors had also suggested, back in 2013, that she have a double mastectomy.
Ms Dawes refused but did undergo surgery to remove what was thought to be a cancerous tumour, as well as lymph nodes from her armpit.
She then had surgery to improve the appearance of her breasts.
She now works in a nursing home and has said of the error: "I am absolutely appalled at what I have been through and am still struggling to comprehend how this could even happen.
"I was very sore after the operation and shocked by the extensive scarring, so to be recalled four days later to be told none of it was necessary was truly horrendous.
"I am still in pain now and the scarring has not improved which affects my self confidence."
Her lawyer, Louise Hawkley, of Irwin Mitchell, added: "We are pleased that the trust has admitted that there were mistakes made during Elizabeth's treatment and we hope that the trust will now work with us to settle the case so she can begin the process of rebuilding her life."