Government probation reforms are "complex and untested", an influential committee of MPs has warned.
The Public Accounts Committee (PAC) has told the Ministry of Justice it must follow closely the progress of its reforms, which involve extending rehabilitation to short-term prisoners and bringing in private providers, to avoid the problems which have haunted previous changes such as electronic tagging contracts.
Margaret Hodge, the Labour MP who chairs the PAC, said the reforms carry "significant risk" because of the short-term programme for the changes.
The PAC has published two reports on the criminal justice system and a "landscape review" of probation.
Prisoner rehabilitation has been thrown into the spotlight in recent weeks with high-profile absconds from open prisons.
Mrs Hodge said: "The provision of rehabilitation services will be extended for the first time to those sentenced to less than 12 months in custody, an estimated 50,000 offenders. This represents a 22% increase on the 225,000 offenders managed by the probation service during 2012-13. However the ministry could not tell us how this significant increase in the case load of probation staff would be managed.
"The ministry must ensure that the current 'good' standard of performance is maintained during the transition to new providers and the increase in offender numbers.
"The new arrangements have not been fully piloted and the ministry expects the new National Probation Service and the 21 Community Rehabilitation Companies to begin operating from 1 June 2014, allowing only a limited opportunity for parallel running of the new arrangements during April and May 2014.
"We therefore welcome the ministry's commitment to only proceed at each stage of the programme if it is satisfied it is safe to do so and that value for money will not be jeopardized."
The PAC has recommended the Ministry of Justice should set out the key review points it will use to assess whether it is safe to progress to the next stage of the programme and report the basis on which, should it decide to do so, it considers it safe to proceed.
It further recommends the performance of the current probation service should be the benchmark against which the reformed service is judged to help make sure standards are maintained.
On the wider criminal justice system, the PAC has said better efficiency and co-ordination is required between departments and agencies.
The committee highlighted a worsening problem of police files handed to the Crown Prosecution Service not including proper case summaries.
Mrs Hodge said: "The remarkably slow progress in improving IT systems over the last decade means there are still too many disparate systems which fail to operate together. There are some 2,000 IT systems in use in the police service, and the Metropolitan Police alone uses over 300.
"The departments need to set a clear vision for future IT with a timetable for how different initiatives will come together to provide a coherent and seamless case management system.
"The Home Office, Ministry of Justice and Crown Prosecution Service have made some efforts to improve efficiency, but these are taking place against a background of reduced resources and staffing, and progress in addressing the key problems remains disappointingly slow."
Justice minister Jeremy Wright said: "Each year there are more than half a million crimes committed by those who have broken the law before, and 50,000 of the most prolific offenders are released on to the streets, totally unsupervised and free to go back to their criminal ways.
"Living with the status quo just means accepting more crime and more victims, and that is not acceptable.
"The public expect us to deal with this glaring hole in the current system and we make no apology for doing so as a matter of urgency.
"These reforms will be rolled-out in a sensible way, to properly protect the public, and we will test our progress at every stage."