Al Qaida supporters are being hardened in British jails rather than reformed, it has been claimed.
Convicted terrorists are using their incarceration to preach jihad to vulnerable young men and build up networks, Whitehall sources told the BBC.
The radicalising process is said to be similar to that experienced by IRA members at the Maze prison in Northern Ireland.
Ministers are trying to tackle the issue by training jail imams, and identifying inmates who may be at the greatest risk.
A Ministry of Justice spokeswoman said: "Protecting the public is paramount and maintaining a secure and safe environment in prisons is a key part of that commitment.
"The Prison Service works hard to tackle extremism in prisons providing guidance to enable governors and staff to deal effectively with any radicalisation in prisons.
"We are actively working with partner agencies, including prison service imams, to ensure that extremists in prison do not pose a threat to the public, prison staff or other prisoners."
