Many offenders are not monitored or supervised properly because there are too many demands on probation staff in England and Wales, a report has said.
The report says there are increasing demands on probation officers
The comments were made by the Chief Inspector of Probation, Andrew Bridges, in the Inspectorate's annual report.
Mr Bridges said the probation service could not meet the public's expectation that all re-offending was preventable.
The Ministry of Justice said it will take his observations seriously and ministers said a review was underway.
This is being conducted to ensure end-to-end supervision of offenders could be sensibly implemented.
Although probation budgets have increased considerably over the past 10 years, Mr Bridges says they have not kept pace with the increasing demands on the service.
The BBC's home affairs correspondent, Danny Shaw, says rising prison populations mean there are more offenders who will subsequently require monitoring.
Complicated supervision schemes like drug treatment orders, prolific offender programmes and the sex offenders register have also increased workload, he says.
Because of this, Mr Bridges says the government's goal of co-ordinated and continuous management of offenders "risks being proven undeliverable".
This in turn, he says, will make it harder to satisfy public expectations about being protected.
He said: "Our overall point here is not a proposal to pour extra resources unthinkingly into the problem but to highlight the contrast, which has developed slowly over a long period of time, between rising expectations and a squeezed capacity to meet them.
"As an organisation with a track record of taking a hard line on the issue of improving quality within existing resources or less, this Inspectorate is able to recognise when efficiency savings year-on-year reach their reasonable limits when demands are still increasing."
Judy McKnight, general secretary of the probation officers' union Napo, told BBC Breakfast that the government's sentencing policy was critical.
"What we have at the moment is a situation where, although overall levels of crime are going down, the numbers in prison are continuing to go up and up," Ms McKnight said.
"What it means is getting to grips with policy to ensure that only those who have to go to prison go to prison."
Mr Bridges said the Inspectorate would continue to support "all feasible measures" to improve its management of offenders.