The public is being put at risk by an increase in the physical and verbal abuse of supervisors of Community Payback schemes, the probation union has said.
Some supervisors, often working for an hourly wage with minimal training, are so scared of reprisals by offenders that they are reluctant to report abuse, Napo said.
The warning over rising levels of violence, including one incident where a teenage offender was shot five times last month, comes as the Government considers plans for a "rehabilitation revolution" which could see more offenders given unpaid work placements instead of short prison sentences.
Harry Fletcher, Napo's assistant general secretary, warned that standards were slipping amid increasing group sizes and a lack of resources and training for staff.
"Unpaid work clearly has an important role to fulfil in sentencing," he said.
"However, if it is to be run by increasingly untrained and intimidated staff, often working alone, and if action isn't taken to decrease the amounts of threats, both verbal and physical, then the public will continue to be at risk. Sessional supervisors are increasingly reporting that they are scared on site and reluctant to report bad behaviour for fear of reprisals. Not surprisingly, there is a high turnover of staff."
In one of the most serious incidents, a 19-year-old man was leaving the community service site at Brooke House Sixth Form College in Clapton, east London, when he was shot five times in nearby Powell Road on July 31. Scotland Yard said two men, in their late teens or early 20s and wearing dark clothes with hooded jackets, were seen hanging around the scene shortly before the attack.
Fears that the attack, and others like it, were gang-related have led to staff being unable to send known gang members to do placements anywhere other than the individuals' own postal districts, Napo said.
In another incident in Merseyside, probation staff received information from a high security prison that one of the inmates claimed he had the means to have a probation officer shot and referred to another offender in the community who would do it. Supervisors also reported calling police after being verbally abused by offenders who refused to carry out the work and one, in Hertfordshire, locked himself in a vehicle for protection after being physically threatened, Napo said.
More than 55,000 individuals were sentenced to unpaid work in the community last year, with about a third being given individual placements, such as in a charity shop, and the rest working in groups, Napo said.