A rape allegation against a known sex offender was not investigated properly in a case "so inept that it borders on the unbelievable", the police watchdog said today.
The Independent Police Complaints Commission (IPCC) found individual and organisational failures at Dyfed-Powys Police.
Four detectives failed to make sure the investigation progressed properly after a woman said she was raped as a child.
The watchdog said the force and the officers involved did not provide the quality of service someone making a rape allegation should expect.
The woman reported the rape in April 2005, alleging it happened 10 years earlier when she was a child.
She appeared as a witness in a trial in 2009 in an entirely different case into a rape allegation against someone else.
The defence questioned her about the allegation she made four years earlier, but she was unable to explain the outcome of the allegation as she told the court she had not heard anything after reporting it.
The court directed the prosecution to find out the outcome of the police investigation. A file on the allegation could not be found, but officers' statements meant the victim's account and credibility were robustly challenged in court.
Further enquiries by police confirmed her recollection of events and a file containing her original statement was found in a CID office in the Pembroke area of the force.
It contained an allegation from another 'victim' against the suspect which had not been recorded as a crime or investigated further.
The suspect had never been arrested and the investigation was never concluded.
The four officers - a temporary detective constable, an acting detective sergeant, a detective sergeant and a detective inspector - have all attended misconduct meetings.
IPCC commissioner for Wales Tom Davies said: "This woman came forward to report an alleged rape that had occurred when she was a child.
"This must have been a very difficult thing for her to have done and she was entitled to a proper investigation.
"The accumulation of individual and organisational failures described in our investigation led to a Dyfed-Powys Police response so inept that it borders on the unbelievable.
"Put simply, at the time that this woman reported the alleged rape to the police the man was a known sex offender on the sexual offenders register and every effort should have been made to investigate the allegation thoroughly. It is a tragedy that it was not."
He added: "These individual and institutional failures were then compounded when two individual officers gave statements to court which led to this young woman facing hostile questioning when she appeared as a witness in a trial for a different case.
"Dyfed-Powys Police has shown us that it has since changed its systems and they believe that the report of a crime such as this could no longer be lost in the system.
"I have been assured by the highest levels within Dyfed-Powys Police that this was an isolated case.
"It is vitally important people can come to the police with confidence that serious matters such as these are properly investigated.
"Failures of this kind though only serve to make the experience more traumatic than it already is and potentially severely dent the public's confidence."
Mr Davies said the force assured him it would deal with such crimes sensitively and thoroughly.
He added that he expected Dyfed-Powys to offer the young woman a full apology.
Deputy Chief Constable Jackie Roberts said: "Firstly I would like to apologise publicly on behalf of Dyfed-Powys Police to the victim in this case for our failure to properly investigate this matter from the outset.
"This was clearly a serious allegation and our response in terms of follow-up investigation was unacceptable.
"We accept the findings of the independent IPCC investigation and the principal officers involved have been subject to misconduct proceedings.
"We are confident that our crime management procedures are now far more improved and robust than was the case in 2005 which will assist in ensuring that the situation that arose in this case should not happen again."