Online paedophiles are being handed short jail terms or even community service and not being rehabilitated, a report found.
The study raised serious concerns about the sentences given to offenders caught with child sex abuse images.
It warned "high risk" offenders were not being locked up for long enough to take part in treatment.
The maximum penalty for possession of child sex abuse at the Crown Court is 10 years in prison.
But the probation officers' union Napo uncovered sentences of jail terms of less than a year, or community punishment.
More worringly, paedophiles were not being made to join sex offender treatment programmes, despite recommendations from probation officers, the report said.
Harry Fletcher, assistant general secretary of Napo said: "In Napo's view it is inappropriate that either no treatment, or sentences too short for treatment, are handed out by the courts.
"There is evidence that treatment in prison, followed by relapse follow up on licence in the community, is effective and reduces reoffending.
"These are not victimless crimes. There is an urgent need to review sentencing practice and law to ensure that risk to children is minimised."
He also warned proposed cuts to probation funding would mean fewer paedophiles going into treatment and a greater risk to vulnerable children.
The dossier contains evidence of 68 cases from 20 probation areas across England and Wales.
In one reported case, an unnamed offender was caught with more than 24,000 indecent images including those at level five, the most serious forms of abuse.
Despite being deemed high risk to children, he was given a 12 month jail term and calls for an intensive treatment programme were not followed.
In another a man was caught with 21,000 images, including very serious abuse, and was given an 18 month term with no treatment attached.
Sex offender treatment programmes are available in 26 out of the 140 jails in England and Wales. There are more than 7,500 sex offenders currently behind bars.
A Prison Service spokesman said around 2,400 paedophiles were treated on its Sex Offender Treatment Programme last year.
He said: "Sentencing in individual cases is entirely a matter for the courts.
"Sex offender treatment programmes are delivered in prison and the community while the offender is on licence or under a community order."Reuse content