Police have lost track of 485 registered sex offenders across Britain including rapists and paedophiles, Sky News can reveal.
Figures released by 41 forces revealed the number of convicted sex offenders whose whereabouts are unknown has risen by more than 20% in the last three years.
Rape, sexual assault and sexual abuse of children were among the crimes committed by the missing offenders, including some who disappeared more than a decade ago.
It comes as victims of black cab rapist John Worboys fight the decision to release him from prison after he spent less than a decade behind bars.
Sky News submitted Freedom of Information requests to the UK's 45 police forces asking for details of registered sex offenders whose whereabouts were unknown.
Four forces did not respond, including the Police Service of Northern Ireland (PSNI), meaning the actual number of registered sex offenders who are missing is likely to be higher.
Forces refused to name the missing sex offenders, with most saying it would breach the Data Protection Act.
Britain's biggest force, the Metropolitan Police, said it did not know the whereabouts of 227 registered sex offenders, including 38 who had been missing for at least eight years.
West Midlands Police said 46 registered sex offenders were wanted or missing – including one offender who disappeared in 2006.
Greater Manchester Police said the whereabouts of 19 registered sex offenders were unknown, with one vanishing in July 2006.
Police Scotland said 12 registered sex offenders were wanted, all of whom are believed to be outside the UK.
Kent and Northumbria Police each had a registered sex offender who went missing in 2005, while Lincolnshire Police said the whereabouts of a registered sex offender who disappeared in 2006 was still unknown.
The total number unaccounted for across the UK is up by 22% compared with March 2015, when 39 police forces revealed they did not know the whereabouts of 396 registered sex offenders.
Duncan Craig, who set up the charity Survivors Manchester after being a victim of sexual abuse, told Sky News that police need to work more with organisations in contact with victims and offenders.
He said: "Victims deserve the right to know that those that have committed these crimes against them are being dealt with correctly and managed properly.
"We can't keep going for the same rhetoric because we won't get anywhere if we do.
"Einstein said the definition of madness is doing the same thing over and over again and expecting a different result. This is exactly what we are doing in this country with management of sexual offending."
Alex Mayes, from the charity Victim Support, said: "These figures will potentially be very alarming to victims of sexual offences and could undermine public confidence in the criminal justice system.
"To ensure the safety and well-being of survivors of sexual offences, as well as local communities, it is vital that the police strictly monitor sex offenders."
Registered sex offenders are required to inform police and probation officers of their addresses so they can be monitored.
They are subject to Multi-Agency Public Protection Arrangements (Mappa), where authorities manage the risk posed by certain sexual and violent criminals living in the community.
A Ministry of Justice report published last October showed there were a total of 55,236 registered sex offenders living in England and Wales in 2016/17.
A Home Office spokesman said the figure of 485 represented less than 1% of the total.
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"We have significantly strengthened the system of reporting that sex offenders are subject to, and a range of civil orders have given police more powers to manage their behaviour," said the spokesman.
"When a registered sex offender goes missing, their details are recorded on national and international systems and the police will actively seek out further information and intelligence to locate them."