Police failed to monitor a convicted rapist who disappeared for nearly a year before he went on to murder the teenager Ashleigh Hall after befriending her on Facebook.

A damning report by the Independent Police Complaints Commission (IPCC) found a crippling lack of resources and inadequate levels of staffing allowed Peter Chapman to "slip away" from the authorities despite supposedly being tracked by Merseyside Police.

Chapman, 34, had a history of sexual offences dating back to when he was 15. He was on the sex offenders' register after being released from prison in 2001 for rape and robbery and over the following two years was questioned twice for further sexual offences.

But, despite being assessed as high risk in 2006, he came under the supervision of a single female officer running the Knowsley sex offender unit on Merseyside. Although she had inadequate training in their management in the community, she was left in charge of monitoring more than 60 offenders and was making visits alone in her own car due to a lack of resources.

The inquiry found that the assessment which led to the 2007 downgrading of Chapman to medium risk was not carried out correctly. Chapman left his home on Merseyside following a hit-and-run incident in 2008 but it took police nine months to circulate details nationwide that he had gone missing.

In October 2009 while posing on-line as a good-looking, bare-chested 19-year-old, he befriended the childcare student from Darlington, Co Durham, before kidnapping, raping and strangling her.

IPCC Commissioner Naseem Malik said: "It is evident that this particular sex offender unit was inadequately resourced and, as a result, the officer tasked with managing sex offenders in the community had an impossible task. Chapman was not monitored effectively and managed to slip away, with terrible consequences." She added: "For it to take a year for him to be apprehended was unacceptable."

The dead teenager's mother Andrea Hall, 41, described the report as "shocking" and "heartbreaking". She said: "If they had done their job properly and managed him properly, and he had not gone missing for a year, he would never have got anywhere near my daughter. I knew the report was going to be bad but this was worse than we thought."

Assistant Chief Constable Andy Ward of Merseyside said the force had strengthened its management of sex offenders since the murder.