Police have lost tabs on nearly 500 registered sex offenders in Britain, according to a report.
And Lincolnshire Police revealed there were four registered sex offenders whose exact locations were unknown - all of which are believed to be outside the UK.
One had been missing since 2006, one since 2014, one since 2015 and another since 2016. Their offences included rape, indecent assault and possession of indecent images.
Figures released by 41 police forces show the number of convicted sex offenders whose whereabouts are unknown is 485, a jump of more than 20% in the last three years.
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.
The figures come as victims of black cab rapist John Worboys campaign to overturn the decision to release him from prison.
Police did not name the missing sex offenders, citing the Data Protection Act.
The total number missing is up 22% on March 2015 when 39 UK police forces said hey did not know the whereabouts of 396 registered sex offenders.
Alex Mayes, from 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 must inform police of their addresses and are subject to monitoring by authorities who manage certain sexual and violent criminals living in communities.
There were a total of 55,236 registered sex offenders living in England and Wales in 2016/17, according to a Ministry of Justice report published last October.
A Home Office spokeswoman said: "The UK has some of the toughest powers in the world to deal with registered sex offenders and we are committed to ensuring that the system is as robust as it can be.
"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.
"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."