The hospital said 355 screenings were reviewed
Eighteen women have been told they have breast cancer after screening errors by a radiologist in East Lancashire failed to detect the disease.
The women were among hundreds who had mammograms at Accrington Victoria Community Hospital, of which 355 cases were reviewed in an independent study.
East Lancashire Hospitals NHS Trust said it was "not possible" to say if the women's prognosis was affected.
The radiologist involved last screened patients in December 2008.
The trust said they had not worked since April.
The blunder came to light through internal monitoring in the screening service which sparked an independent investigation.
As a result all of the radiologist's breast cancer screenings over the past three years were scrutinised, said the hospital trust.
Of these cases 85 women were re-tested and 14 diagnosed with invasive breast cancer and all are receiving treatment.
A further four patients were found to have ductal carcinoma in situ - a non-invasive breast cancer - which had not progressed.
Rineke Schram, medical director of the hospital trust, said the mistakes had caused a delay in treatment.
"It is unfortunately not possible to state with certainty whether this delay in treatment has affected the prognosis other than to state that early-stage breast cancers have a good prognosis."
A helpline has been set up for any women concerned and all of those affected have been informed by the hospital.
Dr Richard Dobrashian, director of breast screening at the hospital, added: "To be told you don't have breast cancer then to be told that you do have breast cancer must be very distressing.
"I would like to apologise on behalf of the breast screening service for any anxiety or distress caused by this."
'Serious management failings'
Dr Alexis Willett, head of policy at national cancer charity, Breakthrough Breast Cancer, called for measures to be put in place to ensure that this situation did not happen again and encouraged women to get screened.
He added: "Any type of misdiagnosis is extremely worrying. It is important to remember that this is a very rare situation.
"The NHS Breast Screening service sees 1.5 million women each year in the UK and it is estimated to save 1,400 lives annually in England alone.
"In the meantime we would encourage anyone with a screening appointment to attend."
Meanwhile, councillors have called for a full investigation to prevent similar mistakes.
Burnley borough councillor Darren Reynolds, a former member of the hospital trust's governors, said: "It must be terrible for them to have to go through this, first being recalled, then being given the all-clear, only to discover in the end that they are affected by cancer.
"There have been serious management failings at the Trust in the past and I do not know yet to what extent this might have contributed to the problem."