Convicted criminals are "not being seen for weeks" and some are being lost altogether because of a failing privately-owned probation service.
The public is being put at risk because thousands of low and medium-risk ex-prisoners are not being monitored properly by the capital's Community Rehabilitation Company (CRC), watchdogs have warned.
HM Chief Inspector of Probation Dame Glenys Stacey said: "Services are now well below what people rightly expect, and the city is more at risk as a result.
"There was some welcome good practice by individual officers and first-line managers but generally, practice was well below standard, with the public exposed unduly to the risk of harm in some cases despite lessons from the past. That is plainly not acceptable."
She added: "A combination of unmanageable case loads, inexperienced officers, extremely poor oversight and a lack of senior management focus and control meant some service users were not seen for weeks or months, and some were lost in the system altogether.
"This simple lack of management attention to basic attendance and supervision was the most striking and surprising finding, and again, not acceptable."
The findings will spark fresh questions about a controversial shake-up of the regime for managing offenders in the community rolled out in 2014.
Under the overhaul probation services in England and Wales were divided into a new National Probation Service and 21 privately-owned Community Rehabilitation Companies (CRCs).
The London CRC, which is owned by MTCnovo, is the largest of the companies and was supervising 28,750 offenders across the capital as of the end of June.
Inspectors examined the work of the CRC and the London Division of the NPS in north London, covering eight out of 32 London boroughs.
On the company's effectiveness in protecting the public, the report said: "The proportion of work carried out to a sufficient standard was low, leaving the public at undue risk."
In one case a man with a long history of offending was recalled to prison following an allegation of domestic abuse, before being re-released subject to post-sentence supervision.
"There was no assessment in place of the risk of future domestic abuse and no flag on the database identifying him as a domestic abuse perpetrator," the report said. "It was not clear where he was currently living and whether or not he was living with a partner."
On effectiveness in reducing re-offending, the report also described the company's performance as poor, adding that there was an "alarming lack of contact" in too many cases.
Justice Minister Sam Gyimah said: "I met senior managers at London CRC and told them this is totally unacceptable. An urgent improvement plan is now in place and I will not hesitate to take more action if necessary."
The Government is carrying out a comprehensive review of the probation system, he added.