Reoffending rates have been slashed in a pioneering police scheme in north-east England which experts say could lead to a transformation in how the justice system deals with violent crime.

More than 2,660 offenders who have committed offences such as burglary and assault have taken part in an experiment in which they avoid prosecution if they take part in a rehabilitation programme for issues such as mental health and substance abuse.

If they complete the four-month contract with police, they walk away without a criminal conviction. If they do not, they are prosecuted in the traditional way.

The initiative by Durham constabulary is the longest-running and most advanced of its kind in England, with at least five other forces, including Surrey and Devon & Cornwall, considering the introduction of similar schemes.

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The first results of the trial, seen by the Guardian, found a 15-percentage point drop in reoffending after two years among those who took part in rehabilitation compared with those who did not.

Most of the 519 offenders in the first cohort of the trial had been arrested for violence, theft, drug possession or criminal damage. About half had no previous convictions but others had been arrested as many as 52 times.

The results, which are part of a University of Cambridge study to be published in 2022, are the most significant indication to date of the long-term success of “deferred prosecution” programmes in England.

The findings will reopen the debate about whether such initiatives represent “soft justice”, as some critics claim, or are successful in reducing crime and cutting costs. The idea has gained traction with senior police leaders and is to be discussed by the National Police Chiefs Council later this month.

Jo Farrell, the chief constable of Durham constabulary, said the current criminal justice sanctions were not effective for dealing with low-level offences such as shoplifting, assault or drug possession.

“What I’m not saying is where people commit serious offences, they won’t feel the full force of the law,” she said. “This isn’t about trying to do things on the cheap or divert people away from court or prison. It’s a cohort of people for whom this cycle will never end unless we do something different.”

Under the Durham programme, called Checkpoint, offenders spend four months with a police supervisor who helps them access support for issues including mental health, to drug or alcohol use, homelessness and communication skills. Of the 2,660 offenders involved in the trial to date, only 166 (6%) have reoffended.

The initiative also involves the victim of the crime, consulting them on the progress of the offender’s rehabilitation, although they do not get the final say on whether the offender is prosecuted. Durham police said only five victims had complained out of the 2,660 offenders who had completed the programme.

Checkpoint costs the force £480,000 a year but an internal estimate suggests that for every 1,000 offenders it saves at least £2m a year in reduced crime.

A similar trial in the West Midlands, which ended in 2014, tracked 414 offenders over two years and found that the amount and severity of reoffending by those on deferred prosecutions was lower, or no worse, than those who were prosecuted. That study, by criminologists at Cambridge, has not yet been published.

Prof Lawrence Sherman of the university’s Institute of Criminology said there was mounting evidence against the prosecution of low-level offences such as theft or common assault. “It’s expensive, it rarely results in a formal punishment, victims aren’t satisfied with it. It’s generally a pretty bad way to manage low-level offences which are the highest volume offences,” he said.

Sophie Gregory, criminology course director at Birmingham City University, described the Durham findings as significant and said: “We know that around two-thirds of women and a third of men are reportedly committing crime to fund addictions, so if we can go back to that root problem and help with some tailored support long term it has got to help reduce reoffending.”

The debate about rehabilitation was reignited by the terrorist attacks in London Bridge in November and Streatham this month, both carried out by men who had been recently released after prison sentences for terrorism offences. Boris Johnson also made “law and order” a key part of his 2019 election campaign, promising among other things to prosecute within a week anyone caught with a knife.

But Farrell said such a blanket approach may not always be appropriate. “Within all offences, there’s context. You get bold statements by politicians: ‘Everybody who carries a knife must go to prison.’ Policing is based on discretion – we have a lot of discretion and a lot of autonomy,” she said.

“Context is everything and the situation you’re dealing with and their background. Taking a very binary position on things strikes me as a not very intelligent way of dealing with an issue.”

Alex Mayes of the charity Victim Support said the one wish of those affected by crime was for the offender to stop offending. He added: “Where evidence shows offender management schemes to be effective at reducing reoffending, many victims of low-level offences may be open to credible and safe programmes.

“However, just as in the wider public, justice means different things to different victims of crime. For this reason, where these schemes are used, it is vital that all victims receive explanations as to their use, and that victims’ ongoing feedback is sought in order to inform the process.”