Prisoners serving less than a year have the highest reoffending rate
Reoffending by thousands of criminals serving short prison terms in England and Wales costs the taxpayer up to £10bn a year, a report has said.
The National Audit Office found many prisoners were spending all day in their cells rather than being engaged in training and rehabilitation.
It added there was also "little evidence" the risk of short-sentence prisoners reoffending had been reduced.
The government said it would take the report's recommendations forward.
Of all those in jail, prisoners serving less than a year have the highest reoffending rates and the most convictions, an average of 16.
Around 60,000 prisoners are jailed for less than 12 months each year, costing taxpayers £300m.
They are mostly convicted of theft and minor violent crimes and at any one time make up nearly one in 10 of prisoners in England and Wales; the prison population is currently about 82,000.
Value for money
Most spend as few as 45 days inside, and are released automatically at the halfway point of their sentence.
But in that time they are not given "appropriate assistance" to help them turn around their lives, the report said.
The NAO found that activities for this group of offenders were "inadequate". About half were not involved in work or courses and spent almost all day in their cells.
The report concludes that short jail terms do not offer value for money. A six-week spell in prison costs £4,500 - £300 more than a highly intensive two-year community order involving unpaid work and rehabilitation schemes.
And overall, with 60% of short-sentenced prisoners committing another crime within a year of getting out, the social and economic cost to the country was between £7bn and £10bn a year.
But John Thornhill, chairman of the Magistrates Association, argued that alternatives to custodial sentences were often simply not available.
He told BBC Radio 5 live: "In my area of Merseyside we have a programme called Intensive Alternative To Custody. Magistrates like using that particular order... but [it] is not available across the country.
"It's only available in six areas and we're told that once the pilot scheme's finished that alternative will not be there."
The president of the Prison Governors Association, Paul Tidball, says magistrates had been criticised for being "so free" with the use of short-term sentences, which did not work.
He told Radio 5 live: "I've heard the Magistrates Association in the past... saying prisons should do more with them [prisoners] while they have them.
"But if you've only got someone for six weeks, there's a limit to what can be done."
He added resources should be concentrated on those at greatest risk of reoffending, and said there should be better investment in communities in the first place to prevent crime.
Former inmate Craig Morrison called for earlier intervention by therapists and more robust community sentences to tackle problems such as drugs.
He said he spent his short stints in jail scrubbing floors and in the gym.
"It was too short a time to do anything rehabilitative with me. You know, I'd just get out and carry on from where I left off, playing catch-up, go out, committing crime straight away," he told Radio 5 live.
"It sounds messed up but I was quite comfortable where I was. I was brought up in care since 12 and I accepted that was my lot in life.
"I had a moment of clarity - unfortunately it was years down the line. I had to go through years of therapy to realise how messed up my life was."
The auditors praised the efforts of the National Offender Management Service (NOMS), which runs prisons and probation, in keeping inmates safe - despite overcrowding.
Former Metropolitan Police Commissioner Sir Ian Blair said it was important prisoners who breached bail or committed offences on bail knew they would be sent back inside.
He supported putting more minor offenders to work in the community on "not very pleasant" tasks that had "an element" of rehabilitation, to free more space in jails.
"It certainly doesn't need to be a chain gang, it can be just a high-glow jacket that shows that this is community punishment taking place," he said.
Amyas Morse, head of the NAO, said achieving NOMS' goal of reducing reoffending by short-sentenced prisoners was "challenging" because so many prisoners spent so few weeks inside.
"However, it is reasonable to expect progress towards that goal. More coherent plans for prisoners, tailored to reducing their risk of reoffending, would be a good first step," he said.
Phil Wheatley, director general of the NOMS, said he welcomed the report and would take the recommendations forwards.
Juliet Lyon, director of the Prison Reform Trust, said: "The revolving door of prison and crime costs the taxpayer billions and does little or nothing to reduce offending.
"The evidence is clear that community penalties, treatment for addicts, mental healthcare and sorting out housing and employment all work better than a short prison sentence."
Jon Collins, of the Criminal Justice Alliance, said rather than spending money on a "futile attempt" to make short sentences work, the government should focus on keeping people out of jail to free up space and resources to better rehabilitate those who needed to be inside.
The Tories said the report was a "damning indictment of Labour's prison failure" and pledged to introduce a "rehabilitation revolution".