Chris Grayling’s decision to part-privatise the probation service is arguably his worst failure. On Thursday, it was thoroughly blasted by the chief inspector of probation, Dame Glenys Stacey. She described the current system as “irredeemably flawed” and said we would all be safer if supervision of offenders in England and Wales was back in public ownership.

More than a quarter of a million people are living under probation supervision – that’s three times as many people as are locked up in prison. Probation staff struggle to cope while our communities are hit with more crime because of a misplaced ideology of privatisation. Probation – just like other vital public services such as the NHS, social care and schools – doesn’t belong in private hands. Privatisation leads to inefficiency, fragmentation and the wrong incentives.

The impact of Grayling’s decision has been utterly devastating. Probation services were working well under public ownership: In 2013, the Ministry of Justice rated all 35 publicly owned probation trusts in England and Wales as good or excellent. In 2015, Grayling, who was then justice secretary, part-privatised our probation service – with no pilot schemes and against huge opposition. This has resulted in probation now being split into two tiers: 70% of cases are low-risk offenders, dealt with by 21 private community rehabilitation companies (CRCs) covering different regions of England and Wales. About 30% of cases are high-risk offenders, which are dealt with by the still publicly owned National Probation Service.

When people are released from prison, probation staff help them comply with the terms of their sentence, find work and somewhere to live, and manage drug and alcohol addiction and mental health problems. They also look after their families and the victims of crime, and offenders who are given community sentences. If Grayling expected innovation from private companies, he got it wrong. What we have actually seen is a “drift away from practice informed by evidence”, according to Stacey. Grayling’s privatisation has put probation workers under “exceptional strain” as, according to Stacey, “capable probation leaders were required to deliver change they did not believe in, against the very ethos of the profession”.

Probation work is about relationships, not transactions and targets. The profession has been diminished, distorted and devalued, according to Stacey, and sometimes “professional ethics have buckled under the pressure”. Staff are hugely overworked, with some handling more than three times the recommended caseloads. There is a staff shortage and too much reliance on agency staff. The inevitable consequence? Eight out of 10 CRCs have been rated “inadequate” for their delivery of casework. The service is buckling and breaking.

‘Making the transition from prison back into society is challenging. Our crumbling probation service makes it easier to fall back into a life of crime.’ Photograph: Sean Smith/The Guardian

This is drastically unfair for everyone involved. It’s unfair, clearly, for probation staff. It’s unfair for the victims of crime. They desperately need proper support and confidence that a strong probation service is keeping a close eye on the situation as perpetrators come back into the community. It’s unfair to ex-offenders, people who have often had an unfair start in life and need help in their reintegration into society. Stacey estimates that one in two had been abused as a child, and about one in four taken into care. Making the transition from prison back into society is challenging. Our crumbling probation service makes it easier to fall back into a life of crime.

It’s unfair for all of us – as our communities are much less safe than they should be. Since privatisation, the number of people committing a serious further offence while under probation supervision has risen by 25%. Privatisation may even be leading to unfair outcomes in our justice system. More people are being sent to prison instead of being given community orders – possibly because judges and magistrates have lost confidence in our privatised probation service.

This is a terrible deal for the public purse – as the National Audit Office recently found that the Ministry of Justice has paid hundreds of millions of pounds more than was required under the original CRC contracts, with various attempts to stabilise and bail out CRCs. And the recent collapse of Working Links, followed closely by Interserve, shows how creating a market in such a vital service can lead to wild instability. As Stacey says: “If probation services are delivered well, there would be less reoffending, fewer people living on the streets and fewer confused and lonely children, with a smaller number taken into care. Men, women and children currently afraid of assault could lead happier, safer lives. These things matter to us all.”

The contracts come to an end in December 2020 – two years earlier than planned. This is a perfect opportunity for thejustice secretary, David Gauke, and the prisons and probation minister, Rory Stewart, to bring them in house.

• Cat Hobbs is the founder of We Own It, an organisation that campaigns for public ownership of public services