Sex offenders who completed a group treatment programme were more likely to commit further similar offences, a Ministry of Justice review found.
Some 10% of criminals who completed the prison-based core sex offender treatment programme (SOTP) committed at least one sexual offence during the period covered by the review, compared with 8% of those who had not received the treatment.
For offences involving explicit images of children, the reoffending rate was 4.4% for those who had completed the programme, compared with 2.9% in the comparison group.
The analysis warned that group therapy could allow offenders to share "contacts and sources" or "normalise" criminal behaviour.
Overall, the review found the core SOTP, which has been used since 1992, was "generally associated with little or no changes in sexual and non-sexual reoffending" but "there were small increases in the sexual and child image reoffending rates".
It suggested that the nature of the programme, delivered to groups of eight men who completed the course over around 180 hours, may have been a factor.
The analysis said it was "possible that attendance on the core prison-based SOTP may increase the propensity to sexually reoffend amongst sex offenders".
"This may have been as a result of the sole emphasis on group treatment", the report said.
One reason could be the lack of individually tailored treatment, but the report added: "Group treatment may 'normalise' individuals' behaviour: when stories are shared, their behaviour may not be seen as wrong or different; or at worst, contacts and sources associated with sexual offending may be shared."
The study recommended that "the treatment approach should be modified in line with the evidence" and "in particular, it could include individual sessions as well as group sessions".
The programme could also "focus more on factors that have been established to predict reoffending".
The study looked at 2,562 convicted sex offenders who started the prison-based programme between 2002 and 2012 in England and Wales who were matched to 13,219 untreated sex offenders as a comparison group.
The study compared outcomes over a period starting from each offender's release up to October 2015, an average period of just over eight years.
The core SOTP was replaced in March, along with the extended SOTP for higher risk offenders.
A Prison Service spokesman said: "We keep treatment programmes under constant review to reduce reoffending and protect the public.
"The treatment of sex offenders is a complex issue, subject to international scrutiny and research. In the UK, we have been evaluating our sex offender treatment programmes since before 2003.
"We have discontinued two of the eight programmes and replaced them with new courses as part of this process. These new programmes will be monitored closely."