Hundreds of convicted sex offenders have gone missing from across the UK, according to police records.
Figures released by 39 forces under the Freedom of Information Act show 396 sex offenders are currently wanted because their whereabouts are unknown.
Sara Payne, whose daughter Sarah was killed by paedophile Roy Whiting, said the figures were "unacceptable".
The Home Office said the UK had some of the toughest powers in the world to deal with sex offenders.
Registered sex offenders are required to inform the authorities of their addresses so they can be monitored.
In FOI responses to the Press Association, police forces revealed the extent of untraced offenders in their areas in February or early March.
The forces stressed the figures could change as arrests were made or new cases came to light.
Among those recorded were offenders who have been missing for more than a decade.
One convicted sex offender in Gloucestershire has been missing since the year 2000, while another in Northumbria disappeared in May 2002.
And Humberside Police said the whereabouts of one registered sex offender had been unknown since September 2004.
The Metropolitan Police, the UK's largest force, said 167 registered sex offenders were wanted in London.
Its figures include one offender who has been missing for 14 years.
A Scotland Yard spokeswoman said London's "diverse multicultural population" meant a large percentage of sex offenders were "either known or believed to be living abroad, having returned to their country of origin".
Last month, the force issued an appeal to trace convicted rapist Patrick Mosekwe Kanda.
He had been living in Dagenham, east London, but failed to report to authorities in February 2013 following his release from prison.
Elsewhere, the Police Service of Northern Ireland (PSNI) said three sex offenders were "currently wanted" but did not reveal when the offenders went missing.
The force said the information might be valuable for "criminal/terrorist/vigilante groups who may attempt an attack on them and/or their families".
Police Scotland said none of the 4,775 registered sex offenders in the country was missing as of 16 February.
Dorset Police had not originally responded to the FOI request but later told the BBC there were three registered sex offenders in the county "who have either failed to say where they are living or who have registered at an address where they are no longer residing".
Humberside Police told the BBC the number of missing registered sex offenders in its area had gone down from seven to six since the Press Association figures were collected.
Greater Manchester Police said the figure had since reduced from 25 to 23.
Sara Payne, whose daughter Sarah was abducted and killed in 2000 by convicted paedophile Roy Whiting, said the figures were unacceptable.
"It's time to take some serious proactive action to bring them back under the police radar," she said.
The NSPCC described the figures as "alarming" and said its own research had found there was just one police staff member responsible for every 50 registered sex offenders.
Jon Brown from the charity said about half of those on the register were offenders who had raped or sexually assaulted children, or had committed offences relating to online child abuse images.
"However, most just receive one police visit a year after they have been released from prison and a period of supervision," he said.
Sharon Girling, a former police officer and founder member of the Child Exploitation and Online Protection centre, part of the National Crime Agency, said police resources were "not there any longer" to properly assess and monitor offenders.
She told BBC Radio 4's Today programme: "I'm slightly concerned because I feel the numbers are always increasing.
"We seem to be getting another five or 10 every year these numbers come out, which means we are not managing these offenders in the appropriate way that the public should expect us to be doing so."
Tagging offenders to follow their movements and tracking their internet use were among the options to monitor them more effectively, she added.
A Home Office spokeswoman said: "The UK has some of the toughest powers in the world to deal with sex offenders and we are committed to ensuring the system is as robust as possible."
She said the Home Office worked closely with forces to ensure legislation was effective and that officers had all the tools they needed.
Registered sex offenders - including rapists and paedophiles - are required to inform police and probation officers of their addresses.
They are supposed to be monitored by officials working under multi-agency public protection arrangements (MAPPA).
Every force to respond to the Press Association refused to name those missing over concerns of vigilante attacks, or because the information was exempt under data protection laws.