Phil Wheatley, who until yesterday ran all 140 prisons in the UK, said that offenders who serve sentences of six months or fewer are not being rehabilitated and on usually go on to commit further offences.
In an interview with The Independent, Mr Wheatley also warned that, with the prison population continually rising against a backdrop of unprecedented public spending cuts, the Government will soon have to decide whether to build extra prisons or start releasing more prisoners early.
There are currently about 8,500 prisoners serving sentences of fewer than six months. Under current legislation they are eligible for release after half of their sentence and, because their sentence is less than a year, they are not given a probation officer.
Mr Wheatley, who leaves his post as director general of the National Offender Management Service (Noms), explained: "If you are using imprisonment to try to change the way someone thinks then you have got to allow time to allow someone to change.
"People who get short-term sentences – and many of them are doing relatively low-level crime like theft and shoplifting to fund a drug habit – often do not have much motivation to give it up. The real question is: are we making them better? Although we are doing our bit, we are not really making a significant difference to the way they reoffend.
"In real terms, if you get a short sentence you would serve half of it in prison. What can I do in, for example, two weeks with a person who is not very well-motivated to change?"
His argument is backed up by figures which show that prisoners released after less than 12 months go on to commit an average of three crimes each in their first year of freedom.
Instead, Mr Wheatley says that there is evidence to show that criminals given restorative justice penalties such as community service have lower reoffending rates than those given short-term jail sentences.
He added: "Anyone who says that short-term imprisonment does not work is perfectly accurate in saying that it does not have a therapeutic effect. Those who do community sentences do better than predicted. Short- term prisoners do worse than predicted. If you are looking for a therapeutic effect, there is not one for short-term imprisonment.
"My consolation is that we have achieved a therapeutic effect in longer-term prisoners and that is a major achievement."
Asked if the Government should scrap short-term sentences in favour of restorative justice penalties, he added: "In certain cases I can see what the thinking is behind it [short-term sentencing]. You could do away with it, but you would have to work out what was the right approach."
Mr Wheatley also warned that the current trend for judges to hand down indeterminate sentences – tariffs which in theory may never end – is stretching resources. He added: "It makes things more complicated because they will obviously only be released if we manage to reduce the risk of reoffending. If they do not prove that they are not a reoffending risk then their chances of getting out are zilch.
"It means that, of the resources we have available to reduce the reoffending risk, quite a lot of it goes on this group, a disproportionate amount. That's because, if we do not spend money on them, they will stay in prison forever – that is not humane to the individual and it is expensive to the taxpayer."
It does however mean that there is less money to spend on the rehabilitation of prisoners serving shorter sentences. Mr Wheatley said this was an unfortunate by-product: "We need to target the money to where it will make a difference. The danger is trying to spread it too thinly. If four people have an illness and 12 tablets will make one person better there is no point giving all four three each; that makes no one better."
Mr Wheatley joined the prison service in 1969, first as a prison officer, then a governor, and became director general of the service in 2003. His departure comes two years after the Prison Service merged with the National Probation Service and he became the first head of Noms.
He says that his decision to leave is due to the fact that, because in 2008 the job was put out to tender after two years, he was effectively being asked to re-apply for his own job, something he was not prepared to do at 62. He is replaced by Michael Spurr, previously the chief executive officer at Noms.
In Mr Wheatley's 41 years he has seen prison population rise from about 35,000 to 85,000 today. The increase, he says, is not down to crime rates, which are falling, but rather the tendency of judges to use prison sentences more frequently and hand down longer sentences.
He said: "At some point someone might decide they want to do something about that.
"It is not a problem for me, the jailer, because it gives me more customers and, just like the manager of Tesco, I do not mind having more customers. But we will need more prisons. If you do not do that you end up having to let people out early.
"The politicians have a choice: do they choose to build or do they choose to find a way of letting some prisoners out early? That is the political choice."
Case Study: 'Offenders don't need punishment: they need support'
Mark Johnson
"I remember the day I was released from my first prison sentence. I was 17 and had just served six months in Portland prison for assault. I was taken off the island on a minibus and put on the train back to Birmingham. I'd reoffended before I even got off the train.
"The first thing I did was buy four cans of Special Brew, take some amphetamine and smoke a spliff. While still on the train I robbed a young lad, took his money off him. The same night I went back to my mum's house and had stolen stuff from her house before I left the next morning.
"Within two months I was back inside after being caught selling cannabis. I did another four months and got out. A few weeks later I was in prison again for stealing a motorbike.
"I was a heroin and crack addict, but the prison service did nothing to rehabilitate me. It was a case of 'lock him up and he'll be gone in a few months'.
"But, in my opinion, I should have never gone to prison in the first place. All of my crimes were committed when I was under the influence of drugs or alcohol.
"I was a young lad in need of help, but I didn't realise it. The courts should have taken one look at me and proscribed some sort of alcohol or drug rehabilitation course. Instead they sent me to prison and somehow expected me to be a law-abiding citizen when I was released.
"The short spells in prison just taught me that I had to take from people or they would take from me. I continued that on the streets and my offending continued and my behaviour spiralled out of control.
"It wasn't until I enrolled myself on an intensive drug rehabilitation programme and realised that punishment was not what I needed – I needed help and support – before I finally realised what I was doing was wrong. But I had to do that myself. No one in the prison service appeared to recognise that."
The author is the founder of Uservoice, a charity involved in offender rehabilitation
