The system of curfews for early-release prisoners is to be extended by ministers in order to remove around 140 inmates from overcrowded jails.

Prisoners who are eligible for release in four-and-a-half months' time are currently allowed out on home detention curfew (HDC), under which they are electronically tagged and must not venture out during specified hours.

Proposals introduced yesterday will extend that to six months, although it will also continue with statutory exclusions from the scheme, such as sex offenders.

Speaking at the opening of new facilities at Perth Prison, Justice Secretary Kenny MacAskill said: "A coherent penal policy must encompass a range of appropriate punishments, including prison for serious and dangerous criminals as well as tough community penalties for low-risk minor offenders."

Bill Aitken, Tory justice spokesman, said yesterday: "Surely the solution would be to build more prisons?"