A serial sex offender admitted kidnapping, raping and murdering a teenager whom he lured to meet him using a false identity on the social networking site Facebook.

Peter Chapman, 33, changed his plea to admit to killing Ashleigh Hall, a 17-year-old student, on the eve of his Monday trial. Hall was strangled and her body was dumped in a field last October, the Times of London reported.

As well as murder, Chapman also pleaded guilty to failing to notify police of a change of address, as required by the sex offenders register.

Chapman has been investigated for six violent sexual attacks but only convicted of two rapes. At the age of 19 he was sentenced to seven years in jail for raping two young prostitutes at knifepoint.

In September last year, he made contact with Ashleigh, who was attracted by a picture of a handsome young man that Chapman had posted on the Facebook.

He pretended the image was a true picture of himself and called himself Peter Cartwright. Ashleigh was one of 173 people to accept him as a friend on the networking site, almost all of whom were young women

The night before Ashleigh’s body was found, she told her mother she was going to stay with a friend, but had decided to go and meet her internet friend.