Up to 25 criminal rape cases could be thrown out of court after it emerged an unregistered nurse carried out forensic examinations on victims for 18 months.
Rape Crisis Network Ireland (RCNI) fear the cases could collapse because forensic evidence gathered by the Health Service Executive (HSE) was by a nurse who had let her membership with the Nursing Board, An Bord Altranais, lapse.
Gardai and health chiefs have informed the victims, who were examined in the specialist sexual assault treatment unit (SATU) in Donegal.
RCNI executive director Fiona Neary said it was a bad day for victims of rape and sexual assault in Ireland.
"We are very concerned for the wellbeing of the victims in these 25 cases, and for the future of child protection in Ireland," she said.
"We know that for survivors of rape the experience of a forensic medical examination can be traumatic.
"Many victims choose to do it because they do not want this to happen to someone else - they want to prevent anyone else being hurt.
"The reality is that for these 25 rape victims, having made the very difficult decision to have a forensic medical examination, the possibility of their gaining justice has been greatly diminished.
"These rape victims have been betrayed by the HSE."
It is understood the nurse was removed from the national register for non-payment of fees in 2009, but her removal never filtered down to the local hospital in Letterkenny.
The Director of Public Prosecutions (DPP) is examining if her testimony and the forensic evidence can be accepted in a court.
A Garda spokeswoman said the HSE has made gardai aware of concerns they have about the registration status of a nurse in their employment at Letterkenny General Hospital.
"As this matter concerns persons who attended the Sexual Assault Treatment Unit (SATU) at Letterkenny Hospital, we are working closely with the HSE to ensure that those involved are notified and are kept fully informed of all developments," she said.
"We have also brought this matter to the attention of the office of the Director of Public Prosecutions."
The HSE said it was notified in 2009 that a group of nurses, including this individual, had been removed from the nursing register as a result of non-payment of fees.
It only became aware in recent weeks that the staff member was still working in the SATU in Letterkenny General Hospital.
Two independent experts have been appointed to carry out a review to establish the circumstances leading up to, and the reasons why, a staff member could continue to practise without being actively registered.
Rosemary Ryan, risk manager at the Irish Public Bodies, and Mary McCarthy, former chief nursing officer at the Department of Health, will also look in detail at any issues relating to professional practice in the SATU.
The investigation began on May 5.
The HSE said: "The review will seek to establish why the necessary processes and checks, in keeping with HSE policy, did not identify that this individual was not registered.
"Since becoming aware of this issue, the HSE has checked, and is satisfied, that all nurses working in other sexual assault treatment units around the country are actively registered."