Police have lost track of 485 registered sex offenders across Britain including rapists and paedophiles, according to a report by Sky News.
This includes 13 registered sex offenders across Thames Valley.
Forces across the UK were asked to reveal the number of registered sex offenders whose whereabouts were unknown on 17 January.
Michelle Skeer, National Police Chiefs' Council lead for the Management of Sexual Offenders and Violent Offenders, said: "The number of recorded wanted or missing sex offenders represents less than 1% of the total and a proportion of these are, following investigation, either known or believed to be living abroad or have returned to their country of origin.
"When registered sex offenders are missing or wanted in the UK, all police forces are alerted. If they return to the UK, there are processes in place to ensure that they are brought to the attention of police and arrested where appropriate.
"The UK has some of the most effective tools in the world to manage registered sex offenders.
"While the reality is that the risks posed to the public by such individuals can never be completely eliminated, there is significant evidence that the multi-agency public protection arrangements successfully keeps them to a minimum."
Here is a regional breakdown of responses from 41 forces. Four did not respond, Sky News said.
Thames Valley Police
The force said the whereabouts of 13 registered sex offenders were unknown.
The force said the whereabouts of three registered sex offenders were unknown.
They went missing in 2014, 2015 and 2016 and the offences were sexual assault, rape and sexual assault of a child.
The force said three registered sex offenders were recorded as wanted because their whereabouts were unknown.
One had been missing for more than a year, while two had disappeared within the last six months.
The force said the whereabouts of one sex offender were unknown.
The force said there were 10 registered sex offenders whose whereabouts were unknown.
One went missing in 2008, one in 2011, three in 2012, four in 2013 and one in 2017.
Offences included sexual assault, attempted rape of girl under 13 and rape.
The force said three sex offenders were missing. Two went missing in 2014 and one in 2016.