The Catholic Archbishop of Westminster says he forgot a priest in his charge had been accused of sex abuse against children.
Cardinal Cormac Murphy-O'Connor - leader of Catholics in England and Wales - acknowledged that he did not send details of the case away for independent review.
Claims had been made that the un-named priest abused a young girl over four years.
The cardinal, who faced calls to resign after appointing paedophile priest Michael Hill, today told Radio 4's Today programme he had sent ten other cases concerning abuse allegations away for review.
But he said: "I must confess for me that is a cause of much frustration, more than frustration.
"I said I gave my word that as far as I could ascertain all the cases where there had been an allegation during my period as Bishop of Arundel and Brighton had been sent away for independent assessment. That I said with great confidence.
"So it is a frustration to me that this particular case, the file and papers were not collated together and it was not sent away.
"I have been away from the diocese for three years. I'm responsible for the files in Westminster and not for the files in my former diocese.
"I am quite sure that now the child protection team in Arundel and Brighton will be dealing with this.
"It is a historic case which according to Nolan (the Church's most recent rules on dealing with such cases) has to be reviewed."
Asked whether he had forgotten about the allegation, he said: "Frankly, I did. Also, I trusted that all the files in my former diocese would be examined.
"I had thought with the Bishop of Arundel and Brighton that we had covered everything. We didn't cover this. Of course, I should have remembered. But of course, the file should have been there. I'm not trying to excuse myself. I'm trying to say that in good faith I said there were 10 cases as far as I could remember, and I didn't remember this one, that is quite true."
The cardinal appointed Hill, a convicted paedophile priest, chaplain at Gatwick in 1985, despite his having been stripped of his licence to work in parishes in 1983 and warnings that he posed a risk.
The 68-year-old was jailed for five years in 1997 for a string of sex offences.
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