The Government must introduce permanent measures to ensure fewer non-violent, low-level offenders are jailed, a crime reduction charity has urged.
Courts should seek alternatives to prison for young offenders, drugs mules from developing countries, people who breach community supervision and vulnerable women, in order to reduce overcrowded prisons, Nacro said.
The charity called for ring-fenced funds to divert mentally-ill criminals from prison to health services and a resettlement initiative to prevent the "revolving door" which sees 70% of short term prisoners reconvicted within two years.
The suggested measures are part of a 10-point plan which also demands legislation requiring courts to take the capacity of the prison system into account when sentencing.
The charity also suggests sentencers are given DVD messages from ministers or the Lord Chancellor highlighting their views to bring about "more balanced sentencing".
In an open letter to Home Secretary John Reid, Nacro's chief executive Paul Cavadino says: "We are concerned that, unless effective measures to reduce the unnecessary use of prison are a permanent centrepiece of criminal justice policy, new prison places could simply be filled by the courts with increasing numbers of new prisoners, providing no long term relief for existing overcrowded prisons."
Commenting on the 10-point plan, Mr Cavadino said: "The Government cannot keep lurching from one crisis to the next, sending confusing messages to the public, which veer between toughness and leniency.
"Overcrowded and unfit prisons struggle to rehabilitate prisoners and do little to protect the public.
"Simply building more prison places will not bring an end to the problem, as any new places could simply be filled with additional prisoners by the courts."
Mr Cavadino said Nacro's proposals would produce a lower and more stable prison population and improve public protection by rehabilitating offenders more effectively.
