Tens of thousands of offenders are being supervised under an “irredeemably flawed” system, a watchdog has warned.

In a highly critical assessment, Chief Inspector of Probation Dame Glenys Stacey said the public could be a “lot safer” than is currently the case.

She flagged up a catalogue of problems including a national staff shortage, sub-standard performance of private providers and shortcomings in efforts to keep victims safe.

Probation services, which manage more than a quarter of a million offenders a year in England and Wales, have been part-privatised for the last five years.

Dame Glenys said public ownership is a “safer option” for the core work.

Her annual report, published on Thursday:

– Reveals white noise is being used to stop private conversations between probation staff and offenders being overheard when meetings take place in open booths;

– Suggests a lack of judicial confidence in probation and community punishments may be leading to more custodial sentences in borderline cases;

– Warns the number of probation professionals is at a “critical” level, with too much reliance on unqualified or agency staff;

– Says eight out of 10 Community Rehabilitation Companies (CRCs) inspected since January last year were rated “inadequate” for implementation and delivery of probation supervision, while the National Probation Service (NPS) is performing better overall;

– Finds the NPS and CRCs should do more to keep victims safe and safeguard children.

Individuals subject to probation include inmates preparing to leave jail, ex-prisoners living in the community post-release and people serving community or suspended sentences.

Under an overhaul known as Transforming Rehabilitation, 35 probation trusts were replaced in 2014 by the public-sector NPS and 21 privately-owned CRCs.

High-risk cases are supervised by the NPS with all other work assigned to CRCs.

The shake-up, introduced under then-justice secretary Chris Grayling, was designed to drive down reoffending but it has been heavily criticised by MPs and watchdogs.

Dame Glenys said: “The probation model delivered by Transforming Rehabilitation is irredeemably flawed.

“Above all, it has proved well-nigh impossible to reduce probation services to a set of contractual requirements.”

She said victims, judges, the public and offenders must all have confidence in the quality of the service.

“I find it difficult to see how this can be achieved while probation remains subject to the pressures of commerce,” Dame Glenys added.

Arguing probation is not a “transactional” service, the chief inspector said: “We are not talking here about who supplies your internet access.

“To be plain, public ownership is the safer option for the core work.

“There’s no doubt about that but it’s all in the detail of the model.

“Public ownership may not work if you don’t get the detail of the model right.”

The MoJ plans to end the existing CRC contracts early, in December 2020.

Under a proposed new system, 10 probation regions would be created in England, with each containing one NPS division and one CRC, while in Wales the NPS would assume responsibility for the management of all offenders.

Dame Glenys said moving to better-funded and structured contracts would help but warned “serious design flaws” would remain unaddressed.

She said she was unable to say whether or not the public is less safe since the introduction of the current regime but added: “What I do know is they could be a lot safer if we get the model right.”

Setting out her probation blueprint, Dame Glenys, whose tenure as chief inspector ends in May, called for a national approach and “evidence-based” services.

At the end of September, a total of 258,157 offenders were on probation in England and Wales – 151,788 under CRC supervision and 106,369 managed by the NPS.

Shadow justice secretary Richard Burgon claimed the “reckless” privatisation has left the public less safe, adding: “The Tories need to act on the mounting evidence, scrap their plans for new private contracts and bring probation back in house.”

Prisons and probation minister Rory Stewart said the report “redoubles my determination to continue working towards a probation service that puts public protection first, commands the confidence of the courts and breaks the cycle of reoffending”.

He added: “Our reforms mean 40,000 more offenders are being supervised, which is a positive move for public safety, but it is clear the current model is not working and there is much more we need to do.

“We have already taken decisive action to end the current contracts early and have invested an extra £22 million a year to support offenders on release – and we are carefully considering how best to deliver an effective probation service for the future.”