Mark Bridger made a confession to a Catholic priest while on remand in Strangeways prison in Manchester, it can be reported for the first time.
Bridger apparently confided in the prison chaplain, telling him he may have disposed of April's body in a river.
Details of his cell-room confession emerged during legal argument in the trial but it was never heard by the jury.
The priest later told police about the admission, which was not made in a confessional.
Bridger's defence team admitted that he was "shocked" when he learned that the priest had spoken to police.
The prosecution decided not to adduce the confession as part of its case.
The River Dovey, which runs through the edge of Machynlleth, was searched a number of times by police and no trace of April was ever found.
Police later played down the "confession", saying it was yet another of Bridger's stories.
The morning after her abduction, Bridger was spotted close to the river carrying a black plastic bag.
He was also eventually arrested close to the riverbank after claiming he had been attempting to help in the search for the missing girl.