Prison authorities in England and Wales have been told to take steps to try to prevent paedophiles who are in custody from setting up rings that on their release could harm children.

For those who have researched the disturbing world of paedophilia, it comes as no great revelation that even when convicted and sentenced to long terms of imprisonment, child abusers continue to pursue their perversion by any means possible while incarcerated.

In that sense it is entirely predictable that they should form paedophile networks in jail, attempt to maintain contact with fellow abusers outside, and make plans to get together once they are released.

What shocks is that until recently, albeit in only a few cases, children have apparently been permitted to enter prisons in England and Wales to visit paedophiles serving sentences, allowing them to be targeted for abuse once the prisoners were released.

''It has been a form of preview as far as the prisoners were concerned,'' says Bernard Gallagher of the department of social studies at Manchester University, who has investigated this unsavoury subject for 10 years. He is author of Grappling With Smoke, a report for the National Society for the Prevention of Cruelty to Children due to be published next month. Gallagher admits he has been shocked by his own findings.

The prison authorities should at least have been alerted to the danger, not least by the not-so-distant revelation that paedophile inmates of a psychiatric hospital in Ashworth abused children brought in ostensibly to visit other patients. His research has shown that the warning signs were there, but ignored.

In Scottish prisons, they are known as ''beasts''. In England and Wales they are ''nonces''. Considered the lowest of the low, they are segregated in prison for their own protection.

In Scotland, the authorities reckon they are on top of the situation. Currently, around 300 sexual offenders serving four years or more are held in Peterhead Prison. No distinction is made between rapists and child abusers.

A handful of others are being held in local prisons such as Barlinnie, Perth, Shotts, and Saughton, some of them awaiting transfer to Peterhead.

However, the evidence in this forthcoming report of what currently happens south of the border is worrying. The numbers of those imprisoned in England and Wales for child sexual offences has grown many-fold in the past 10 years. By and large they are devious individuals who mix only with their own and make plans to continue their life patterns once they are released.

Gallagher came across three paedophiles who met for the first time in prison and then formed a ring to travel to Romania in order to abuse children. It was not a one-off situation. ''While it might not be a problem in Scotland at the moment, any trend in England has to worry us,'' says Derek Turner, general secretary of the Scottish Prison Officers Association.

If the authorities did not know it before, they now know they cannot be complacent in Scotland. While cigarettes and drugs are the normal currency in prisons, already cases have been uncovered north of the border where convicted child abusers have been selling transcripts of their court cases to fellow paedophiles.

The thrill they get from reading these legal documents reminds them of life they might look forward to when released. Only in July, two sex offenders in a Scottish prison wrote to one of their male teenage victims shortly before they were due to stand trial at which he would be a key prosecution witness.

Some years ago all letters from prison were censored. European rules now forbid this except in the case of Category ''A'' prisoners. Similarly all inmates are allowed to make telephone calls to the outside.

These European rules make it easier for the abusers to maintain links with their fellows on the outside, prison staff complain. Although it will be some weeks before the 40-page report for the NSPCC is published - and it deals with the growing problem of paedophiles outside as well as inside prison walls - the authors made known their early fears to prison authorities.

As a consequence, governors in England and Wales were issued with new guidelines in May. The first rule laid down was that children would be allowed to visit only close relatives.

Governors have been told that closer attention has to be paid to addresses on outgoing mail, without actually breaking the law by opening letters, and soon smart telephone cards will be introduced which will allow calls to be made to authorised numbers.

The Genie is out of the bottle.

In the old days, prison was seen as a university course in crime for first offenders. Now the same can be said for paedophiles.

''It is a myth that sex offenders do not pose a risk from inside prison,'' says Gallagher.