Cuts to probation programmes for offenders will lead to thousands more crimes, the probation union Napo has warned.
The programmes, which include schemes to target violent or sex offenders as well as those with drug or drink abuse problems, can reduce reoffending by between 8% and 35%, research has shown.
But cuts of up to 15% to probation budgets over the next three years will lead to fewer programmes being available, more offenders being sent to jail and higher costs to the criminal justice system, Napo said.
It comes as under-fire Justice Secretary Kenneth Clarke prepares to outline his plans for a "rehabilitation revolution" after being forced into a U-turn following public outcry over his proposals to halve sentences for those who plead guilty at the first opportunity.
Napo said the financial pressures will mean more offenders will be given community orders that do not include any programmes, which can increase the cost by about £2,000 in each case.
Harry Fletcher, Napo's assistant general secretary, said: "The introduction of probation programmes is patently a success story. However a 10-15% cut to probation budgets over the next three years would lead to a reduction in the number of programmes that can be delivered.
"Firstly because the service will lose experienced staff and secondly the programmes are more expensive than supervision without requirements, but the reconviction rate for those without requirements rises to 50%."
Mr Fletcher went on: "It is therefore clear that a reduction in programmes, which is already happening, will add several thousand to the prison population, the reconviction rate will rise for those who do not participate and this will lead to thousands more crimes.
"The way forward is for the courts to be encouraged to make short community sentences of no more than six months and that programmes be delivered intensively, with three not one attendance on a course per week. This could be delivered at half the cost of incarcerating the same individuals and will have a beneficial impact on reoffending rates which then means less victims, crime and cost."
A Probation Service spokesman said: "Public protection is our main priority and there is no evidence to suggest that savings within the service will compromise probation programmes. Any savings will look to retain frontline services which will ensure the public is protected and re-offending is reduced."