Stuart Hazell has been jailed for a minimum 38 years for killing his partner's granddaughter, Tia Sharp.
The 37-year-old had denied murder, however on the fifth day of his trial at the Old Bailey he changed his plea.
The court heard Hazell sexually assaulted and murdered Tia, 12, at her grandmother's house in south London.
Judge Justice Nicol said he could not impose a whole life tariff as he was not sure Hazell's motivation was sexual or that it was pre-meditated.