Thousands of sex offenders are failing to complete rehabilitation programmes before they leave prison, the NSPCC has warned.
The NSPCC says more must be done to protect children
The child protection charity said the government must do more to ensure the offenders - including many paedophiles - receive proper treatment ahead of their release.
It made the demand after Home Office figures released on Monday showed the number of sex offenders being monitored in the community had risen 16% to 21,413 in the last year.
NSPCC Policy Advisor, Christine Atkinson, warned that prison staff need more "support, training and resources" to make sure more offenders undergo treatment.
The NSPCC said the Prison Service's annual report showed the target of at least 950 prisoners completing the Sex Offender Treatment programme was not met.
And it said a 2002 inspection report on Whatton Prison, which deals exclusively with sex offenders, showed that only 47 of the 272 prisoners had completed a behaviour programme.
The Home Office figures showed that 17 paedophile crimes were committed by offenders being tracked in the community last year.
Among them was a man preparing to abduct a
child. He was subsequently charged with intent to rape a child and jailed for 13 years.
Ms Atkinson said: "There are some good community initiatives working to reduce the risk of sex offending.
"However all opportunities to treat and rehabilitate sex offenders need to be utilised. These opportunities are being missed in prison."
The NSPCC's demand for more to be done in prisons was backed by the Victims of Crime Trust.
Its director, Norman Brennan, told BBC News Online: "If you put rehabilitation programmes together for sex or any other offences and you clients do not complete them then the rehabilitation is doomed."
Mr Brennan said sex offenders who refuse to complete the course, often from contempt for the system, should not be considered for early release.
The Home Office said it was looking at ways to get more prisoners to complete the courses - which force offenders to confront their behaviour and change their ways.
But a spokesman said the success of the treatment depended on the willingness of offenders to cooperate.
He said: "In terms of whether you can force someone to complete a programme, you can take a horse to water but you can't force it to drink."