Thousands of people are not completing their Community Service Orders because of staff shortages within the Probation Service, a union says.
Probation staff shortages can mean people are sent to prison
Harry Fletcher, assistant general secretary of Napo, says the situation is an "extremely worrying problem".
He said there were more than 29,000 cases of offenders sent home this year through staffing or logistical issues.
The Ministry of Justice said investment and staff in the Probation Service had increased since 2000.
Napo represents more than 9,500 people working in the National Probation Service as well as the Children and Family Court Advisory and Support Service.
Mr Fletcher said that being stood down - sent home when there is no work for an offender, or not enough staff are available for supervision, or there is no transport - can affect those who are ready to carry out work in their communities.
"Being stood down demotivates people. Two-thirds of people who are on CSOs have got problems with drugs and alcohol.
"If we send them home after the have taken the trouble to turn up then that will demotivate them."
Ministry of Justice figures show 29,852 work appointments were cancelled in England and Wales between 1 January and 30 June this year. A total of 644,697 were kept, leaving a cancellation rate of 4.6%.
Mr Fletcher said inadequate staffing levels meant that there were currently three problems occurring with CSOs.
"One, courts are being advised that we cannot supervise people, and therefore offenders are more likely to be sent to prison.
"Two, offenders go into a queue until there is somewhere available that can take them on so they can begin their CSO.
"Or three, the CSO commences but the offender is sent home instead."
Napo recently asked its members if they were experiencing problems with having to stand down offenders.
One probation officer replied to say the situation in their service regarding stand downs "has been very depressing of late".
They said: "We have gone from a unit that rarely stood anyone down to a unit that regularly stands people down.
"Last year we stood down very few offenders. This year nearly 60 have already been stood down. The main reason for this is the service's policy of not replacing staff."
Mr Fletcher said the consequence of frequently standing offenders down was that more people are likely to be put into a prison system which is already "bursting at the seams".
He added: "The problem is that since the creation of the National Offender Management service, and the regional structure that's underneath, these organisations have sucked up all the money for the Probation Service, which is quite small and only has about 20,000 staff.
"More money is spent on the national and regional bureaucracy than on the service as a whole.
"The government has to ensure as a matter of urgency that there are sufficient staff and placements to meet the demands of the courts.
"The courts will become increasingly upset because their intentions are being thwarted."
A Ministry of Justice spokesperson said: "Since 2000, staff in the Probation Service has increased by approximately 5,000 and the increase in the probation resource budget, in real terms, over the last 10 years is approximately 70%.
"The National Offender Management Service (NOMS) monitors the proportion of days of unpaid work that are lost and works with probation areas to tackle any issues.
"The average unpaid work hours received by an offender in 2005 was 120.6 and the probation service ensures this order is completed.
"In England and Wales during the quarter April to June 2007 only 4.8% of unpaid work days were lost as a result of operational difficulties."