Criminals are being allowed to miss meetings with their probation officers if they claim they overslept, or had no transport to get there.
Community orders are promoted as punishment outside prison
The "unacceptable" excuses have been revealed by an official watchdog, the National Audit Office (NAO).
The sessions were part of supposedly tough community order sentences imposed by courts as an alternative to prison.
Justice Minister David Hanson said action was underway to address the issues raised in the report.
The community orders - which involve a variety of activities including unpaid work, drug rehabilitation and alcohol treatment - have been promoted by ministers as a tough alternative to sending offenders to jail.
In 2006, courts in England and Wales handed out 121,700 community orders but the NAO said that probation service still need find out exactly how effective the orders are at preventing re-offending.
The Probation Service's guidelines recommend that offenders who fail to attend components of their particular order twice in one year are sent back to court.
Auditors from the NAO looked at almost 700 excuses which had been accepted by probation officers in the five areas covered by the service in England and Wales.
In 9% of the cases they examined, the criminal involved claimed they had forgotten, were "confused", or had overslept.
In London, 19% of the excuses accepted by the probation officers fell under this category.
In 11% of cases, criminals were able to miss meetings by producing a self-certified sick note.
The NAO report said that sometimes officers needed to be more "rigorous" in forcing offenders to turn up.
The report also revealed that the National Probation Service did not know how much community orders cost the taxpayer, or many orders it has the potential capacity to deliver.
The probation service was also criticised in the report for not having complete data on the number of offenders who failed to complete the orders they had been sentenced to.
A study of case files by the NAO showed that 6% of offenders had been unable to finish all of their requirements before the order ended.
Edward Leigh, chairman of the Commons' all-party Public Accounts Committee , said: "Too often, offenders who have been given a community order are escaping elements of their punishment and rehabilitation.
"This is because the probation service, which manages offenders serving the orders, often fails to ensure all elements of an order are completed before it expires.
"In fact, the service does not know how many orders have not been completed, nor how many offenders break the terms of their orders.
"It doesn't know how much community orders cost to impose, nor how many orders it can handle.
"This lack of grip is highlighted by the fact that some probation officers are accepting an offender's oversleeping as a valid reason for his or her failure to attend probation appointments.
"The law-abiding taxpayers of this country deserve better."
NAO chief Sir John Bourn said although there is some evidence that community orders can reduce the likelihood of reconviction, he was concerned by the report's findings.
Ministers say more staff and extra money will solve the problem
He said: "The service needs to identify its capacity to deliver community orders and the associated costs, and the effectiveness of different order requirements in reducing reconviction.
"As a matter of urgency, the service should establish a mechanism to monitor and report the number of orders not completed in accordance with courts' wishes."
But Mr Hanson said the report highlighted the committment that probation officers had to their "challenging" jobs.
He said: "They enforce sentences rigorously and there is evidence of positive outcomes achieved by community sentences."
He added: "Tough community sentences that effectively address offenders' behaviour can get right to the heart of the offending.
"The sentences provide punishment and restrict liberty for individuals in order to change patterns of behaviour in often chaotic lifestyles.
"We have noted the contents of the report, have accepted all the recommendations, and actions are already underway to address all of these."
The goverment says that more probation staff have been hired, more guidance provided to them, and an extra £17 million has been found for the service's budget for 2008/2009.