A paedophile named on a list of the UK's "most wanted" was arrested but later released after police failed to recognise him.
Joshua Karney was photographed after his recent arrest
An inquiry is under way after South Yorkshire Police officers only realised who Joshua Karney was after he was fined for being drunk and released.
Karney, 30, is one of five of the UK's most wanted sex offenders after going missing from Lancashire in 2005.
South Yorkshire Police said they greatly regretted releasing Karney.
He went missing three years ago after serving a prison sentence for indecently assaulting a child in Barnoldswick, Lancashire.
Karney was on licence but failed to report to a probation hostel in Blackburn.
The hunt for Karney has appeared on BBC One's Crimewatch and he is on the Child Exploitation and Online Protection "most wanted" list.
When he was arrested for being drunk and disorderly in Barnsley on 24 November he gave false details and was given a fixed penalty notice.
Karney was released and it was only later that officers checked his fingerprints and realised he was wanted.
A spokesman for South Yorkshire Police said: "We regret greatly that Joshua Karney was released from custody after he gave false details.
"South Yorkshire Police are working together with Lancashire police to trace Karney.
"An investigation is now under way into the full circumstances to establish how this happened."
Lancashire police have released a picture of Karney taken when he was arrested in Barnsley in November last year in a fresh appeal to find him.
Det Insp Jim Elston, of Lancashire police, said: "We are working closely with South Yorkshire Police to establish the background circumstances behind his arrest and his movements within South Yorkshire area at the time."