The Scientologists are persuading prisoners to take courses in the teachings of their late guru L Ron Hubbard. The inmates complete question papers in their cells and send them to the Church of Scientology for marking.
Probation officers and prison reform groups are alarmed at the development, which comes at a time when prison rehabilitation programmes are being cut back.
Stephen Shaw, director of the Prison Reform Trust, said: "Prisoners are often vulnerable to the promises of cult religions. Everyone has a right to practise their beliefs but the Home Office must make sure that Scientologists are not using prisons as a recruiting ground."
Nico van den Berg, a Dutch lawyer who has set up the Scientologists' Criminon UK project said that 16 prisoners, in five jails, were undergoing the programme.
He said that it was intended to expand the scheme next year with Criminon volunteers going into prisons to take rehabilitation classes.
Mr Van den Berg added: "Once we become bigger we can actually go into prisons which might adopt the course as an official programme which all prisoners go through."
He said that Criminon was not seeking to recruit the inmates to Scientology but merely trying to turn them away from criminality by introducing them to The Way to Happiness, Hubbard's secular teachings on clean living.
The suggestion was greeted with suspicion by rehabilitation professionals. Harry Fletcher, deputy general secretary of the National Association of Probation Officers, said: "There are now 57,000 people in prison, many of whom are desperate to change their lives. You can see why the Scientologists would see this as a fertile recruiting ground. This is one service we could do without."
Other critics have been more damning, suggesting that Scientology is a dangerous cult which uses brain-washing techniques on its followers.
In a 1984 High Court judgment, Mr Justice Latey, described Scientology as "corrupt, sinister and dangerous" and "grimly reminiscent of the ranting and bullying of Hitler and his henchmen".
The cult is now developing a different public image thanks partly to the way it has been embraced in Hollywood. Travolta's last film, Phenomenon, has been described by some critics as a thinly disguised piece of propaganda for Scientology, which claims to have 8 million members worldwide.
Meanwhile, senior prison service sources said that Lord McNair, the Liberal peer, was lobbying Michael Howard, the Home Secretary, to allow Scientology services in jail.Reuse content