The public has been left at greater risk of harm because of the poor performance of a privately-owned probation company responsible for supervising thousands of criminals in the community, watchdogs have warned.
Some offenders were not seen for weeks or months and some were lost in the system altogether, according to a highly critical inspection report.
HM Chief Inspector of Probation Dame Glenys Stacey said probation services in London have deteriorated.
She said this was largely due to the poor performance of the Community Rehabilitation Company (CRC), which is responsible for managing low and medium-risk offenders in the capital.
Dame Glenys 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."
The assessment of risk of harm posed to others, and subsequent planning, was not carried out well enough in over half of the cases inspected.
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.
Work started to bring about improvements was welcomed by the watchdog.
Helga Swidenbank, director of probation at the London CRC, said: "We recognise the importance of independent inspection and appreciate we have a number of improvements to make.
"Our change plan, introduced prior to the inspection, is already addressing the recommendations made in the report and is more far reaching as it identifies and tackles the many legacy issues inherited from the London Probation Trust.
"The change plan has a clear objective to reduce re-offending and protect the public across London."
She described public protection as the company's number one priority, saying: "Since the inspection, we have continued to make significant progress in reducing case loads managed by our staff, prompt enforcement, and quality of offender supervision.
"We have already taken steps to ensure that every single case is being actively managed to further protect the public. It's important to recognise that only 40 cases were inspected, just 0.13%, of London CRC's case load."
She added: "We have committed significant investment into London CRC including staff training, new ICT systems and bringing in new management disciplines."
Inspectors found the NPS was delivering services better, but with plenty of room for improvement.
Mayor of London Sadiq Khan said the "deeply alarming" report "shows that London's probation service is simply not working".
He said he will continue to make the case to the Government that responsibility for probation services in London should be devolved to City Hall.
Shadow justice secretary Richard Burgon said: "The Justice Secretary must admit now that privatisation of the probation service has been a failure."