“Our role is not to punish. The punishment is the prison sentence: they have been deprived of their freedom. The punishment is that they are with us,” says Nils Öberg, director-general of Sweden’s prison and probation service.

Öberg, 54, is giving the annual Longford lecture on penal reform in London tomorrow, where he will explain how, in stark contrast to the UK, Sweden is closing prisons and reducing the prison population.

Since 2004, Swedish prisoner numbers have fallen from 5,722 to 4,500 out of a population of 9.5 million, and last year four of the country’s 56 prisons were closed and parts of other jails mothballed. In contrast, the prison population in England and Wales is 85,000 out of a population of 57 million.

With reoffending rates at about 40% – less than half of those in the UK and most other European countries – does he attribute this success to the country’s effective policies on prisoner rehabilitation? “We obviously believe that it is part of the explanation; we hope we are doing something right. But it’s going to be very difficult to prove that scientifically. We are increasing our efforts all the time,” he says.

Last year a “national client survey” of several thousand Swedish prisoners was undertaken in order to identify the issues that have affected their criminal behaviour. “The survey did not bring up any surprises, but it gave us confirmation of what we have learned from experience – that it is not one problem that our clients face, but two or more, sometimes as many as seven or eight different ones, including perhaps drugs, alcohol and psychiatric problems. And these problems did not just appear overnight. These are things that have developed over years. Most of the sentences in this country are relatively short. The window of opportunity that we have to make a change is very small, so we need to start from day one. Our strategy is to cover the whole range of problems, not just the one problem.”

Sweden’s high-security Kumla prison Photograph: Marc Femenia / TT/Press Association Images

Unlike England and Wales, where since 2004 anyone convicted by the courts is categorised as an offender, the implication in the Swedish model is that sentenced individuals are still primarily regarded as people with needs, to be assisted and helped. As well as having rehabilitation at the heart of its penal policy, the other huge difference between the Swedish and UK approaches is the role of politicians.

Chris Grayling, the justice secretary, has recently introduced measures that amount to “a ramped-up political emphasis on punishment rather than real rehabilitation” in prison regimes, according to Juliet Lyon, director of the Prison Reform Trust. These include forcing prisoners to wear uniform, banning books being sent to prisoners, and turning off cell lights at 10.30pm in young offender institutions.

Öberg says: “A politician who tried something like that in Sweden would be thrown out of office. It would be a breach of our constitution – in our system that is the forbidden area. When we exercise authority over individuals, a politician cannot interfere with the administration process. In reality, there is a dialogue – politicians will tell me and my colleagues what they expect and we will do our best to achieve those goals. We have a very clear division of labour between the government and public administration.

“An individual politician cannot interfere with the running of our business. The government sets goals in a yearly letter of intent, and then the responsibility for the work is entirely ours.”

But what about public opinion in Sweden? Is there less desire for retribution than in the UK? “There is a lot of anger among the Swedish public when it comes to crime and criminals,” says Öberg. “But, regardless of what public opinion may be at any one time, whatever you do in the justice sector, you have to take a long-term perspective. You cannot try something one day and then change it to something else the next day – that would be completely useless. The system in our sector is set up to implement long-term strategies and stick to them.”

He adds, however, that the country’s well-educated population appreciates that almost all prisoners will return to society. “So when you go into a political dialogue, there is a fair amount of understanding that the more we can do during this small window of opportunity when people are deprived of their liberty, the better it will be in the long run.”

Is he hoping his Longford lecture will provide some helpful advice that may assist the UK government with its prison difficulties, ranging from overcrowding, staff shortages and a 69% increase last year in self-inflicted death?

“I’m very excited to be giving the lecture. But I will be very careful about giving anybody any advice. We can try to share our experiences and perhaps inspire each other a little bit, recognising that the preconditions for carrying out our work are very, very different.

My ambition is to try to tell a story about how we have come to the conclusions that we have, and explain why we have made the choices that we have made. It has to do with whether you decide to use prison as your first option or as a last resort, and what you want your probation system to achieve. Some people have to be incarcerated, but it has to be a goal to get them back out into society in better shape than they were when they came in.”

Curriculum vitae

Age 54.

Lives Stockholm.

Family Married, three children.

Education Sollefteå Gymnasium, Sweden; Harvard University, Kennedy school of government, USA; political science degree; Uppsala University, Sweden, MSc/Phd, political science.

Career 2012–present: director-general, Swedish prison and probation service; 1999–12; various director-general roles, Swedish ministry of justice; 1995–99; civil servant, Swedish commissioner, European commission, Brussels; 1991-95; officer and analyst roles, Swedish security service; 1988–90: consultant, International Foundation; 1986-88: administrative director, Alva and Gunnar Myrdal’s Foundation: 1983–85; consultant, Swedish Red Cross and other NGOs.

Interests Sailing.