Research for the National Society for the Prevention of Cruelty to Children shows paedophile networks are being allowed to form within prisons, where abusers are segregated from other inmates.
Some jailed paedophiles have managed to establish contact with outside abusers they know and children have been brought in to them. It was suspected that the children were to be abused by the prisoners on their release and that the visit was a "preview."
The Prison Service said it had taken comprehensive action after being alerted to the danger of children visiting sex offenders last May.
It will extend its use of Smart phone technology which will let prisoners dial only approved numbers.
"We only now allow visits [to paedophiles] from their own children," Martin Narey, director of regimes, said. "We do not allow any other children to visit them. We routinely monitor correspondence and we only allow them to make calls to certain registered numbers."
He admitted, however, that in the past it could have been possible for children to have been brought in to visit paedophiles who were not their parents.
"As soon as we had any indication that this might be happening, we moved very quickly to prevent it happening in future," Mr Narey said.
In a preview of sections of the NSPCC report, Grappling With Smoke, to be released in November, the charity says 240 organised sex abuse rings may be under investigation at any one time.
The research, by Bernard Gallagher of Manchester University, an expert in organised sex abuse, highlights two cases where abusers in the community wrote to abusers in prison.
One, adopting a false name, visited paedophiles in prison. The contacts helped in the setting up of new rings and "promoted" existing ones, and thwarted police investigations through methods including the planned harassment of witnesses in impending trials.
The report is intended to help social workers, police and others involved in dealing with organised sex crimes.Reuse content