The leader of Roman Catholics in England and Wales says a "filing error" led to him to fail to pass on details of a priest accused of child abuse to an independent investigation.
The cardinal is spiritual leader to four million Catholics
Letters seen by BBC Radio 4's Today programme show that in 1993 Cardinal Cormac Murphy-O'Connor was informed about allegations a priest had sexually abused a young girl over a four year period.
But he failed to send the details to an independent solicitor as he had promised he had done in "every" such case.
The cardinal, who is Archbishop of Westminster, told Today he had "forgotten" about the case which allegedly involved a 12 year old.
I felt desolate - I felt like no-one would believe me
Alleged child abuse victim
Last December, he said that he had sent the files on 10 cases brought to his attention when was Bishop of Arundel and Brighton.
"I was concerned to see that every case where allegations had been made was assessed by a professional," he said.
On Thursday, he told the BBC he had acted "properly and professionally".
"In good faith I said there were 10. But I didn't remember this one."
The girl involved, who claims the abuse continued until she was in her late teens, says the church made her feel like a "12-year-old whore".
"I felt desolate. I felt like no-one would believe me, no-one would help me," she said.
Priest still working
She was able to show the BBC letters sent by her own priest to Cardinal Murphy O'Connor outlining her accusations and a letter from the cardinal to her.
The cardinal said the priest was investigated at the time and social services and the police decided not to take the allegations further.
He said the priest, who is still working in a church attached to a school, was assessed by a psychologist and "the conclusion was he was not a risk to children".
BBC correspondent Angus Stickler met the priest and put the allegations to him.
He admitted he had "fondled" the girl but thought it was when she was above the age of consent.
The church admits that it cannot account for its copies of the confidential letters produced by the alleged victim and the cardinal said this was "frustrating".
"It is one of my sorrows that any failings in the system seem to indicate the Catholic Church does not care about child abuse - this is not the case," he added.
The archbishop is the spiritual leader in England and Wales for more than four million Catholics.
He was made a cardinal by the Pope in February 2001.