She said Fr John Lloyd raped and indecently assaulted her on numerous occasions as she sat on his lap in a darkened room.
"It happened every time I went to confession - once a week - over a period of about 18 months," she told Cardiff Crown Court. "I thought about telling my mother, but I was afraid. I thought I would be in trouble if I told her."
The woman, who cannot be named, was the first prosecution witness to give evidence at the trial of Fr Lloyd, 57. He denies sexual assaults on six children aged from eight to 13 over an 18-year period, while working as a parish priest in South Wales.
The woman explained that Fr Lloyd regularly visited her primary school to hear the confessions of the children, who queued up outside a room partitioned off with black curtains. "We all went through one by one, about four of us went at lunchtime," she said.
She said the children would knock on the door and wait to be called in by Fr Lloyd, who sat in a chair. They knelt in front of him, made their confession and then received a penance of prayers to say.
She said the priest sat her on his lap and then put his hand inside her blouse and groped her private parts. Frequently he also penetrated her and had sexual intercourse, she added.
"That happened many times to me, numerous times. I would guess more than 20 times," she said.
The woman wept as she told the jury how after the assaults she would lock herself in a school lavatory and try to clean herself up.
"I felt so dirty and ashamed," she added.
She had not told anyone at the time. Fr Lloyd would just tell her she was "special".
"I didn't tell anyone what was happening. Initially, I thought it was affection, but then I realised and I became ashamed," the woman told the jury.
The prosecution alleges that Fr Lloyd also indecently assaulted altar boys who served at his church.
Police investigations began in 1996 after one woman came forward to make allegations against the priest 25 years later. Fr Lloyd, of St Joseph's, Penarth, near Cardiff, is currently on leave from his duties.
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