The British government is hoping that up to 30,000 prisoners can be released early from prison under a new electronic tagging scheme.

Three private contractors have been selected by the Home Office as preferred bidders to run the scheme which is due to come into operation in January.

Tagging involves fitting a small electronic device to the wrist or ankle of an offender and installing a monitoring device at the place of curfew. If the offender is not within the range set by the conditions of the curfew (usually the perimeter of a home) during the curfew hours, the monitoring equipment reports the violation to a central monitoring service.

Violations can lead to the curfew being revoked and the offender being returned to court or to prison.

The preferred bidders are Premier Prison Services for the Midlands/Wales and London/eastern regions, Securicor Custodial Services for the northern region and General Security Services Corporation for the southern region.

Bidders awarded contracts in September will initially be required to provide electronic monitoring services for the home detention curfew scheme and for pilots of court-ordered electronically monitored curfews.

Under the home detention scheme, introduced by the 1998 Crime and Disorder Act, selected short-term prisoners will spend up to the last two months of their sentence in the community, subject to a curfew enforced by electronic tagging.

Tagging is in use in England and Wales under 1991 legislation which allows courts to impose electronically monitored curfews on offenders aged 16 and over.

Trials of tagging have been carried out since 1995 in Berkshire, Greater Manchester and Norfolk. More than 1,500 curfew orders have been made with more than 80 per cent of offenders successfully completing their sentence.

The government intends to extend electronically monitored curfews as a community sentence throughout Britain once the home detention scheme has bedded down.