More than four out of five offenders in probation hostels pose a high or very high risk to the public, figures showed today.
Terrorists, murderers, rapists and child sex offenders were being housed in the approved premises across England and Wales, the probation union Napo said.
Harry Fletcher, Napo's assistant general secretary, warned that as higher risk offenders were now being sent to such hostels, budget cuts and proposed privatisation were "bound to compromise public safety and increase the risk of reoffending".
Almost 90% of the offenders were subject to multi-agency public protection arrangements (Mappa) and at least 19 offenders were regularly monitored by police, the union's study found.
Of the 455 men studied in more than a fifth of the hostels in England and Wales last month, 374 were deemed to pose a high or very high risk and 399 were being monitored by Mappa.
This included 183 sex offenders, 216 men convicted of violent offences, and four other dangerous offenders.
A further 24 were convicted of arson, terrorist offences or drug dealing, 18 were persistent prolific offenders and 10 were on bail.
"Sex offenders and those convicted of violence and terrorism must be supervised at the highest level possible," Mr Fletcher said.
"It is of deep concern therefore that the Government is cutting probation hostels' budgets and considering privatisation."
Cuts of 15% over the next four years and introducing a "profit motive" with private providers "will undermine supervision and compromise the relationship between the police, probation and other statutory agencies", he said.
Probation hostels are "a vital resource for probation's public protection strategy", he said, adding that it was far better for offenders to be in hostels than dispersed in the community.
Over a 24-hour period, an average of 11 staff were working in each of the 23 hostels studied, with each housing an average of 20 offenders.
Mr Fletcher added the profile of residents in hostels has changed dramatically over the last 15 years as it has "moved away from being those on bail to those released from custody following either determinate or life sentences".
The survey of 23 of the 101 hostels in England and Wales last month revealed details of some of the residents.
* In the North West, a 72-year-old man who murdered his wife, dismembered her body and placed body parts in a freezer, was released from jail to the hostel after serving 19 years of a life sentence. He was said to see his offence as "isolated" and does not mix well with other residents.
* In the South East, a 41-year-old arsonist is still considered to pose a risk of "endangering life, including residents of the hostel".
* In the North East, a 40-year-old man was described as "dangerous and a high risk to staff", leading the hostel to use an extra member of staff during the day due to the risk he poses, and all meetings with him are carried out in pairs.
* In Greater London, a 27-year-old man involved in training camps in Afghanistan has prompted staff concerns about his "ideological perspective".
* In Yorkshire, a 40-year-old sex attacker is "dealing with his alcohol consumption and persistent use of cannabis".
* In Merseyside, a 24-year-old robber is "connected to local gun gangs and identified as high-profile by the police".
* In Wales, a 46-year-old man who breached a sex offender prevention order "has distorted belief and thinks he has the right to have sex whenever he wants to".
* In the West Midlands, a 45-year-old rapist was in a hostel after being released from jail after serving an indeterminate sentence for public protection.
* And elsewhere in the Midlands, a 42-year-old man convicted of indecent assault against a boy aged under 13 was described as "narcissistic and egocentric with limited victim perspective". There were also fears he may try to groom other residents.
A Ministry of Justice spokesman said: "The purpose of Approved Premises has for many years been to supervise high risk offenders in the community, a number of which are already run effectively by independent organisations. All providers must meet the same rigorous standards.
"No final decisions have been made on competing out Approved Premises and there is no evidence to suggest that savings made to the Probation Service will compromise public protection.
"Approved premises are designed specifically to protect the public and this will remain paramount in the future. Offenders must follow a structured regime, which includes an overnight curfew.
"Approved Premises have fully-trained staff on-site 24 hours a day and CCTV. Staff work very closely with offender managers and with the Police through MAPPA any concerns about offenders' behaviour are reported."