Children's charity the NSPCC is calling for tougher restrictions for sex offenders who live in the community.
The NSPCC wants to extend offender surveillance
The charity says the stringent conditions applied to those who pose the greatest concern should also be applied to all "medium risk" offenders.
The NSPCC also wants more funds for the police and probation service to monitor sex offenders in England and Wales.
The Home Office said sex offenders were "tightly managed" and resources were focused on public safety.
The call from the NSPCC comes as ministers prepare to reveal the number of sex offenders under surveillance in the community.
Wes Kewell, the director of services for children at the charity, told BBC Radio Five Live: "If you're a medium risk offender you'll be expected to report from time to time, but otherwise, basically, you will be left to get on with it, with severe warnings about your conduct.
"And what it really means is there is a high risk they could be deteriorating in their behaviour, they could be beginning to track potential victims."
Currently high-risk sex offenders in England and Wales are monitored by police and probation officers under arrangements known as multi-agency public protection arrangements (Mappas).
They are subjected to strict licence conditions on release from prison, and can be sent back to jail if they fail to co-operate. They can even face covert surveillance by detectives.
The NSPCC wants to see Mappas given more resources so these stringent conditions can be extended to all medium-risk sex offenders.
The charity says the probation service in particular is understaffed and under-resourced.
A spokeswoman for the Home Office said all sex offenders, at all levels of risk, were tightly managed.
She added: "The probation service takes seriously the management of all sex offenders under its supervision.
"Through Mappas, violent and sexual offenders are being identified and better managed than ever before."
The sex offender register meant the police were kept informed of their whereabouts, she said.
"The probation service, working closely with the police and prison service, aims to ensure that resources are focused where they can have the biggest impact on public safety."
The assistant general secretary of probation union Napo, Harry Fletcher, said he estimated the NSPCC's demands would involve the monitoring of an extra 20,000 offenders, at a cost of about £25m.
"The Mappas are an excellent way of protecting the public and evidence shows they work well," he said.
"But the constant complaint from staff is that they are under-resourced, and an expansion such as this would need to be properly funded to reflect what could be a tripled workload."
The NSPCC has also called upon the government to publish an annual report on the monitoring and supervision of child sex offenders across the country.
It would include statistics on the re-offending rates of all violent, sex and other dangerous offenders.
Last year, a report by Mappas showed there were 24,572 on the sex offender register in England and Wales compared with 21,413 a year earlier.
Statistics also revealed 26 offenders monitored under the scheme had committed serious sexual or violent offences while on the programme, a drop of 46% compared to the year before .
The number of high-risk offenders referred to the multi-agency panels last year was 2,152.