Thousands of offenders living in communities are managed by a brief phone call every few weeks, a watchdog report has revealed.
In some cases convicts are only met once before being placed on "remote supervision" by private probation providers.
This can involve speaking on the phone to an officer every six weeks or so with no further face-to-face meetings taking place.
Inspectors said the conversations amounted to little more than "checking in" and raised concerns over whether the approach would detect any change in the risk posed to the public.
The revelations were detailed in a scathing critique of the Government's controversial shake-up of the system for managing offenders in England and Wales.
Known as Transforming Rehabilitation, the £3.7 billion partial privatisation launched in 2014 saw the creation of the National Probation Service to deal with high-risk cases, while remaining work was assigned to 21 community rehabilitation companies (CRCs).
In her annual report, Chief Inspector of Probation Dame Glenys Stacey flagged up how some individuals managed by CRCs do not meet with their probation worker face-to-face.
Instead, they are supervised by telephone calls every six weeks or so from junior staff overseeing 200 cases or more.
In one instance, a man given a community order after being convicted of supplying class A drugs was charged with an offence of wounding while he was being managed by phone calls.
Another man with more than 30 previous convictions for a range of offences, including domestic abuse, was supervised through telephone contact only on release from prison.
Some CRC operating models allow up to four in 10 individuals to be supervised remotely, according to Dame Glenys's assessment.
She acknowledged that the arrangements are allowed under the terms of the contracts, but emphasised that face-to-face work is vital.
Dame Glenys said: "I find it inexplicable that, under the banner of innovation, these developments were allowed.
"We should all be concerned, given the rehabilitation opportunities missed, and the risks to the public if individuals are not supervised well."
CRCs are responsible for the majority of 260,000 offenders being supervised in the community.
The Government anticipated that most of the cases handled by the companies would be assessed as lower risk, according to the report.
But in practice inspectors found that two in three are deemed to be medium risk, requiring more resource and effort than was initially envisaged.
Dame Glenys also found that:
- Reform has created a "two-tier and fragmented" system in which private companies are performing significantly worse than public sector elements;
- Unexpected changes in sentencing, severe financial stresses and cutbacks, and IT failings have undermined the CRC's ambitions for innovative approaches;
- The quality of CRC work to protect the public is "generally poor";
- Resettlement services are providing little help with housing, jobs, addiction and debt, with one in 10 people released without a roof over their head;
- In too many cases individuals on community or suspended sentences are not seen often enough and there is too little purposeful activity.
Dame Glenys said: "Regrettably, none of government's stated aspirations for Transforming Rehabilitation have been met in any meaningful way.
"I question whether the current model for probation can deliver sufficiently well."
Richard Garside, director of the Centre for Crime and Justice Studies, said: "So dysfunctional have the Government's probation changes become that active sabotage would look much the same.
"Whatever the Government's original intentions, its changes to the probation system have not worked."
Shadow justice secretary Richard Burgon said the report "highlights what a colossal failure the Conservatives' part-privatisation of probation has been".
The Ministry of Justice said: "In some cases, lower-risk offenders can be supervised by telephone after a thorough, face-to-face risk assessment, and their continued suitability for this type of monitoring will be kept under review."
Justice Secretary David Lidington said he has made probation a priority.
He added: "It is reassuring that the NPS - which supervises high-risk offenders - is doing a good job overall and we will use this incisive report to continue improving it.
"We have already changed CRCs contracts to better reflect their costs and are continuing to review them.
"We are clear that CRCs must deliver a higher standard of probation services."