Twenty women have undergone unnecessary mastectomies, while others suffered significant harm while under the care of an NHS trust, a review of cases has found.
Hundreds of patients have also come to harm during treatment in the breast unit at the County Durham and Darlington Foundation Trust (CDDFT).
The trust said it was reviewing breast cancer cases between January 2023 and February 2025, alongside those of 640 former patients who contacted a helpline.
Of the 514 cases examined so far, 315 patients were found to have suffered harm, including 77 who suffered significant harm, it confirmed to the Press Association, while one patient is known to have died.
Some cases have also been identified where cancer diagnoses were missed or delayed, leading to the spread of disease.
Denise Howarth, 51, of Consett, who is being represented by Hudgell Solicitors, was diagnosed with stage one cancer in her left breast in February 2023.
She had a lumpectomy and lymph nodes removed, but at a follow-up review was told she still needed a mastectomy.
The mother-of-two underwent the mastectomy, followed by chemotherapy and radiology treatment.
In September last year, she was told the mastectomy had not been needed.
She said the call "knocked me off my feet" and she was left "completely shocked".
"An apology is not enough. This is life-changing," she added.
"I've lost so much confidence. I feel robbed of the person I was before. As a woman, losing a breast has a huge impact on you. I fear I will never be the same person again, although I have tried to be strong."
A board meeting of the trust next week will decide whether the review should be extended to look at thousands more cases dating back to 2015.
A report by the Royal College of Surgeons (RCS) last year, seen by the Press Association, was published by the trust.
It highlighted "outdated practices within the breast surgery service with the potential to cause physical and/or psychological harm to patients".
It also pointed to "evidence of excessive or unnecessary surgery in some cases".
Rachel O'Connor, who is part of the medical negligence team at Hudgell Solicitors, has called for a public inquiry.
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The trust confirmed to the Press Association that consultant Amir Bhatti was CDDFT's clinical lead for breast services from 2013 to 2024.
His clinical practice is currently restricted while an internal trust investigation is carried out, a spokeswoman said.
She added that the trust has been co-operating with Durham Police. The force is looking into what happened at CDDFT.
The National Crime Agency (NCA) told the Press Association that its major crime investigative support unit is assisting Durham Police.
Steve Russell, who was appointed CDDFT chief executive last year, said in a statement: "We are deeply sorry to the women and families who have been harmed and let down by failings in our breast services.
"We recognise the profound impact this has had and have been clear that there were opportunities when, as a trust, we should have listened more carefully and acted sooner.
"Significant changes have since been made to the breast service and are bringing improvements for patients.
"Alongside this, the look-back continues to review past care, identify patients who may have experienced harm and make sure they receive appropriate information and support."
The trust has been the subject of a BBC investigation into its care.
In a statement to the broadcaster, Mr Bhatti said: "I am grateful for the opportunity to respond, but I remain committed to the ongoing trust investigation, and it would be inappropriate for me to discuss this whilst proceedings are ongoing."
Sky News has sought a response from Mr Bhatti.
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