Sex offenders remain a risk to the community for longer than previously thought and much longer than other types of criminal, claims new research.
A quarter of convicted sex offenders get another conviction, a study suggests
A 21-year study showed a quarter of the 419 men released from prison in 1979 received another sexual conviction.
A fifth of those waited at least 10 years before sexually offending.
The report published in the Journal of Legal and Criminal Psychology raises questions about whether current supervision periods are sufficient.
The study sample consisted of all adult male sexual offenders discharged from prison in England and Wales during 1979.
Of the 419 men, 103 were reconvicted for a sexual offence in the next 21 years.
Reconviction rates (%)
2 years: 43 men (10.3)
5 years: 66 men (15.8)
10 years: 84 men (20.0)
21 years: 103 men (24.6)
*Source: Sexual offenders report
Some waited beyond 10 or 15 years before reoffending.
The report questions whether those who do not reoffend should be supervised in the community for longer than the current maximum 10 years.
It says: "These findings raise questions about the appropriate length of supervisory periods for sexual offenders released into the community."
A Home Office spokesman said this would have considerable time and cost implications.
He also said sex offenders now go through prison treatment programmes and are subject to stricter monitoring when released.
"Under the Criminal Justice Act, offenders who pose too great a risk to be managed in the community can now be given indeterminate sentences so they can be kept in prison," he said.
Donald Findlater is deputy director of the child protection agency, the Lucy Faithfull Foundation.
He told BBC News Online: "What may surprise the public is the rate of reconviction is low, relative to other crimes."
But he warned offending rates were inevitably much higher because of the difficulty in securing convictions and the number of unreported sex crimes.
"The report is saying we have to take a long-term view of risk and make sure there are resources to manage and assess that.
"But we know treatment programmes can reduce that risk and we must invest more in them."
Mr Findlater is well aware of resource shortages in this area.
The Wolvercote Clinic in Surrey, which he managed, was the UK's only residential treatment centre until it closed in 2002.
Non-reoffending rates were estimated at 80%.
Dr Ute Navidi, Head of Policy at ChildLine, said: "Everything possible must be done to ensure people who have committed sexual offences against children are unable to commit further crimes.
"If this research is right, and up to a quarter of sexual offenders released from prisons reoffend, and that the risk persists for many years, it seems that supervision, particularly for those assessed as posing a high risk should be supervised for a long time, where necessary more than 10 years."
The Stop It Now! campaign operates a free helpline - 0808 1000 900 - giving advice and support to friends and families of people who have, or show signs of, offending sexually against children.