A prison originally designed to house 450 inmates is to expand to accommodate 1,000 - including some taking part in sex offender treatment programmes.
Wayland Prison opened in 1985 to house 450 inmates
New pre-fabricated cells for up to 300 extra prisoners are being moved into Wayland Prison in Norfolk.
The jail has a reputation for being run well but government inspectors reported prisoner numbers were causing strain.
Independent Monitoring Board's Norman Simpson said he was confident the jail could cope with the latest expansion.
"Wayland has expanded three times over the past 10 years from 450 to 750 which is the current capacity," he said.
"In each previous case, the staff and management have dealt with the growth efficiently and well so there is no reason to believe the latest expansion will be just as successful."
The prison opened in 1985 and is a category C prison providing vocational training for inmates.