The system for supervising criminals in England and Wales is in a "mess" after a government shake-up failed to meet its aims, MPs have warned.
In a highly critical assessment, the Justice Committee said it was "unconvinced" probation reforms can ever deliver an effective or viable service.
Ministers overhauled the arrangements for managing offenders in 2014 in a partial privatisation known as Transforming Rehabilitation.
The National Probation Service was created to deal with high-risk cases, while remaining work was assigned to 21 community rehabilitation companies (CRCs).
The project aimed to reduce reoffending rates but it has repeatedly come under fire from MPs, inspectors and spending watchdogs.
Publishing the latest assessment, Conservative MP and Justice Committee chairman Bob Neill said: "Transforming Rehabilitation was designed by the government to improve the way offenders are managed in the community and reduce reoffending.
"The TR reforms had some laudable aims but these reforms have failed to meet them. We are unconvinced that TR will ever deliver the kind of probation service we need."
There are "major questions" to be answered on a whole range of issues, he said, including the support people get when they leave custody, the performance of probation providers, contracts with CRCs, poor staff morale and the involvement of the voluntary sector.
Mr Neill added: "This has a negative impact on the number of individuals who go on to reoffend. Hard-working and dedicated staff are doing their best with a probation system that is currently a mess."
The committee called on the Ministry of Justice to carry out a review of the long-term future and sustainability of delivering probation services under the system, publishing the findings by February.
The report also stressed that telephone reporting should not be the only means of supervision, following revelations last year that thousands of offenders living in the community are managed by a brief phone call every few weeks.
Prisons and Probation Minister Rory Stewart said: "This was a significant programme of reform.
"For instance, an additional 40,000 people who would not previously have been monitored now receive support and supervision upon release.
"Fewer people are re-offending and there have been some innovative and impressive programmes.
"However, we accept that there have been challenges and it is clear that CRC services do need to improve.
"We are currently in commercial discussions with providers and will consider all possible options to ensure we deliver this improvement."