Government plans to make every convict leaving prison spend at least a year under supervision in the community have been branded "unworkable" by a leading campaign group.
Justice Secretary Chris Grayling has announced all offenders who enter prison, even for just a few days, will be subject to the new policy.
Each will be given support into housing, employment, training and substance abuse programmes.
But Mark Gettleson, from the Howard League for Penal Reform , said there was no money to pay for the changes, which would merely exacerbate a failing system.
"We think it's wholly unworkable and the wrong solution to the problem," he said.
"It's an admission of abject failure on the part of the Government with regard to short-term prison sentences. It is clear from every piece of evidence ever presented that times in custody of less than a year are a total failure."
Mr Gettleson described the 58% reconviction rate for short-term prison sentences as "dismal" - compared to 34% for community sentences, which he said came in at 10th of the cost of jail terms.
"Rather than ... actually looking at alternatives that work, or that work a lot better, like community sentences, the Government is going to bolt on a period of unfunded probation support to those failing sentences," he said.
The reforms, to be rolled out across England and Wales by 2015, will see around 65,000 offenders, serving sentences of up to two years, receive extended rehabilitation.
The changes form part of the Government's so-called 'rehabilitation revolution', which will see a greater role for private and voluntary sector organisations, who will be paid by results to reduce re-offending.
Mr Grayling said: "It is simply not good enough that we spend £4bn a year on prisons and probation, and yet make no real dent in the appetite of offenders to commit more crime.
"It is little wonder when many of our most prolific criminals leave prison totally unsupervised in the community.
"These reforms are essential and will ensure that offenders are properly punished but also given targeted support to help them turn away from crime for good."
Shadow Justice Secretary Sadiq Khan said: "This is another wasted chance by the Government - it should have pursued a model that focused on evidence of what works to reduce re-offending.
"Instead it has obsessively pursued pet projects leading to the imposition of an untried and untested payment by results model on the probation service which will take big risks with public safety and taxpayers' money."