Fifty women "are likely to have been harmed" by a consultant gynaecologist, a hospital trust has said.
An investigation into Daniel Hay for University Hospitals of Derby and Burton NHS Trust (UHDB) began in 2019.
Now an interim report has found there is "major concern" over 50 cases and "some concern" over a further 69.
UHDB has apologised to those "identified as being harmed" by Mr Hay, who retired due to ill health in July 2020.
Mr Hay "became unwell" in July 2018, the trust said, and after colleagues who took over his patients raised concerns a review was started in March the following year.
The investigation of 58 cases found "13 areas of concern, including poor documentation, insufficient information on the rationale for clinical decision making and some operations added to the waiting list that were not necessary or could have been avoided".
In December 2019 the trust contacted NHS England "to request oversight of an independent external investigation", which looked at 327 cases involving outpatients and women undergoing major procedures such as hysterectomies.
The reviewers found cases where they "questioned the need for the chosen surgery or its timing or whether other conservative options might have been appropriate".
They also identified "instances where relatively young women underwent sterilisation or a hysterectomy at their request but without the benefit of a referral for a second opinion".
In two cases where reviewers said there "was evidence of harm", one involved a cancer that "might have been detected earlier" and another concerned "possible damage to a fallopian tube" during an abdominal surgery procedure called adhesiolysis.
Analysis: Rob Sissons, BBC East Midlands health correspondent
It is possible the numbers of cases where it's thought women have been harmed will increase as more give their testimony.
The scope of the investigation is currently confined to 2015 to 2018, but Daniel Hay was working for years before that, and some are asking if his work was of the required standard then.
One solicitor says she has women going back to 2005 asking questions about their care.
This is a time-consuming exercise, and undoubtedly the Covid crisis has slowed things, but for women with questions going around in their mind, clear answers are wanted.
'Areas of concern'
The report made seven recommendations, including advising UHDB to audit its record-keeping, review "policies and procedures in relation to staff health and wellbeing" and "consider the advisability" of reviewing other cases.
UHDB has previously admitted at least eight of Mr Hay's patients had been harmed, and last month it was announced the trust would be writing to 383 women involved in the review to offer them the chance to speak to a consultant about their care.
A final report is due in 2022.
Magnus Harrison, UHDB's executive medical director, said work to implement the report's recommendations is "either currently under way or will start shortly".
"When colleagues initially raised concerns in the latter half of 2018 and right through to today, we have worked diligently to piece together a picture of the care they received under this consultant in an open and transparent way," he said.
"I want to make clear that the standard of care these women received was far below that which we strive to provide, and for that I am very sorry."