About 1,200 prisoners are to be freed later ahead of their scheduled release date as part of the government's attempt to ease prison overcrowding.
The early-release scheme aims to free space in the prison system
The offenders are being let out up to 18 days early. More than 25,000 could benefit after inmate numbers in England and Wales topped 81,000.
But inmates convicted of serious sexual or violent offences are excluded.
Newly-appointed Justice Secretary Jack Straw told the BBC the early release scheme would continue for "some time".
"This will carry on until we do get stability in the prison population," Mr Straw told BBC Radio 4's Today programme.
The releases come as a report into Portland young offenders institution in Dorset suggested "damage" caused by prison overcrowding.
The early-release scheme, the first of its kind for 20 years, was announced by former Justice Secretary Lord Falconer last week.
The aim is to free space in the prison system and end the costly practice of holding prisoners in police stations and court cells.
Those serving less than 12 months will not be supervised but could be sent back to prison if they re-offend during the early-release period.
Mr Straw was appointed Justice Secretary in Thursday's reshuffle
Probation officials have expressed concern about whether the offenders will be properly risk-assessed.
Mr Straw asserted that prison doors were not "simply being opened".
"What we are talking about here is people to be released 18 days before their normal release date," he said.
Chief Inspector of Prisons Anne Owers said the Portland report was a clear example of the damage caused by the prison population crisis.
An unannounced inspection found the institution continued to suffer from unfit buildings, insufficient activity, negative staff culture and inadequate arrangements for young people's safety.
The young people, mainly from London, were also held too far away from their homes, it found.
Chief Inspector of Prisons Anne Owers said: "In spite of its continuing difficulties, Portland had improved and managers were well aware of the scale of the task ahead.
"But that task was greatly compromised by the effects of population pressure."