Public safety is at risk as huge workloads and staff shortages continue to place the probation sector under pressure, inspectors have said, while officers lack the “professional curiosity” needed to spot potentially dangerous behaviour among offenders managed in the community.

Her Majesty’s Inspectorate of Probation has painted a picture of a service in crisis with hundreds of vacancies, overstretched officers and managers, and crumbling, overcrowded buildings, including hostels for recently released offenders.

An inspection of the central functions supporting the National Probation Service (NPS), the publicly run arm of the probation sector, which supervises about 105,000 high-risk offenders, found that some officers were not properly reviewing their cases.

The report follows a number of high-profile cases with links to probation, including those of the London Bridge terrorist Usman Khan and the serial rapist Joseph McCann.

As part of their work, probation officers are tasked with rooting out offenders who claim their behaviour has improved but have not actually reformed – known as “false compliance”.

Khan, a convicted terrorist who killed two people on London Bridge in November while out of prison on licence, appeared to have convinced professionals around him that he was no longer a risk by taking part in rehabilitation courses.

Inspectors said false compliance was a real challenge for probation officers who needed highly tuned skills and expertise to interview and challenge offenders effectively. Justin Russell, the chief inspector of probation, said the lack of skills could be down to inexperience in the job.

Amid questions over the failings in the case of McCann, a convicted burglar who carried out a string of sex attacks after being freed in a probation error, Russell said probation officers were still not carrying out basic domestic violence and child safeguarding checks for some offenders.

The inspectorate said having a workload that offered “enough space and time to do that reflective thinking” was important.

The NPS has a workforce including 6,500 probation officers and a budget of more than £500m a year. Inspectors rated all of its divisions as requiring improvement on staffing, and none of the areas are fully staffed. High rates of staff sickness average 11 days per person, 50% of which relate to mental health difficulties, and there are 650 job vacancies nationwide.

Russell said the vacancies were in part related to a pause in recruitment from 2014, when disastrous privatisation reforms introduced by the then justice secretary Chris Grayling were being pushed through. Those reforms are in the process of being unravelled after the Ministry of Justice (MoJ) decided to renationalise the sector.

Those probation officers in place had an average of 39 cases on their books but could have up to 60 at any one time, while victim liaison officers had an average of 215.

Russell said staff shortages were especially acute in London and the south-east. He said putting probation officers under such pressure could compromise effective work with offenders and their ability to manage cases properly.

Staff were being “driven spare” by offices in disrepair, he added. Inspectors found broken CCTV, plumbing and heating problems and one case of a rat infestation. Russell said it was unacceptable that outstanding repairs also meant premises approved for housing high-risk offenders when they were initially released from prison were out of action.

The inspectors also found significant areas of positive performance, including better services for victims and women under supervision. Leadership was good but middle managers were too stretched, the report found.

Among 24 recommendations made to HM Prison and Probation Service – the part of the MoJ that runs the NPS – department bosses missed opportunities to make improvements, inspectors said.

Russell said immediate steps should be taken to tackle workloads and more investment in training was needed.

The justice minister, Lucy Frazer, said: “We know that probation is not getting enough of the basics right – that’s why we are bringing all offender management back under the National Probation Service, which the independent inspectorate says is good at protecting the public.

“It is also clear that the workload is simply too high for many probation officers and the 800 new officers currently training to join the NPS will make a real difference. I am reassured that the chief inspector shares my confidence in the vision and leadership of the National Probation Service – which will be essential to delivering these reforms.”