The Prime Minister's "tough but intelligent" approach to a rehabilitation revolution is doomed to failure, the prison officers' union has warned.
David Cameron said all but a small number of high-risk prisoners will receive help to turn their lives around and break the cycle of reoffending by the end of 2015.
But his plans to get tough on crime and rely on private firms and payment-by-results schemes were "doomed to failure", the Prison Officers' Association (POA) warned.
"The Government has closed prisons, announced increased prison capacity, and still more than 50% of our prisons are seriously overcrowded," a POA spokesman said.
Juliet Lyon, director of the Prison Reform Trust, said: "Payment by results is a rather scary experiment and rolling it out across the justice system without evidence of its effectiveness presents considerable risks.
"The risk with tough talk is that it almost always drives up prison numbers, making it harder for prisons to serve as an effective place of last resort in our justice system."
Mr Cameron's long-awaited speech on crime and justice comes after the Chief Inspector of Prisons Nick Hardwick said politicians wanting a rehabilitation revolution faced a stark choice - "reduce prison populations or increase prison budgets".
He highlighted conditions at Britain's largest jail, Wandsworth prison in south-west London, where excellent workshop facilities stood empty while prisoners watched TV in their cells because there were no resources to move offenders from one to the other.
Vicki Helyar-Cardwell, director of the Criminal Justice Alliance, which represents almost 70 organisations, also said prison overcrowding and budget cuts meant that rehabilitation efforts were being undermined.
Harry Fletcher, assistant general secretary of the probation union Napo, added: "The coalition's policy of cuts, combined with payment by results, is undermining effective practice. It will prove impossible to measure or monitor cause and effect in both prisons and the community."