Unemployment has started to fall after 18 months
Job centres were ill-equipped to support government back-to-work schemes in the face of the recession, the Tories have claimed.
They say a report written for ministers shows the depth of the "crisis" in the job centre network was partly due to the number of offices closed in 2008.
Staff were "unprepared" to provide the extra help promised to jobseekers, it found, while offices were understaffed.
Ministers said service improved quickly and £5bn had been invested in centres.
Figures out on Wednesday show the number of unemployed people in the UK has fallen slightly.
The Office for National Statistics said that total unemployment stood at 2.46m for the three months to December - a drop of 3,000 on the previous quarter.
Ministers say government intervention has helped stem the rise in jobless totals and that 450,000 fewer people were out of work than had been feared a year ago.
However, they have warned that unemployment levels could rise again given the fragile nature of the economic recovery.
But the Conservatives say support for the unemployed has been hampered by staffing, administrative and computer problems in Jobcentre Plus offices.
They cite a recent report, written for the Department of Work and Pensions by the Policy Studies Institute, as evidence that staff were ill-equipped to deal with putting new back-to-work schemes into action.
The report analysed the initial roll-out last April of two separate government initiatives aimed at helping the newly unemployed and those out of work for six months to search for work and get training.
After speaking to job centre staff and jobseekers, the researchers found that high demand for these initiatives had put a "strain on resources" and their rapid introduction had led to delays in services.
"Jobcentre Plus offices were largely unprepared for the extra demand for services they were experiencing," the report said.
"All the observed offices were perceived to be understaffed, strained further by high staff turnover."
Although the report noted that staff levels improved, it said that new recruits lacked experience while some existing staff struggled to come to terms with their new responsibilities.
One worker quoted in the report said she was initially "horrified" at having to deliver several new services at once but after five months she grew in confidence and was "fine with it".
The report said these problems were exacerbated by other shortcomings such as building work in job centres and failures in the computer system.
However, the report also concluded that most jobseekers believed the support they received was well tailored to their needs and jobs found for them were a "good match" for their skills.
The Conservatives said 500 job centres had closed since 2002 while an office a week shut in early 2008 at a time when unemployment levels were already rising.
"The knock-on effect has been that employees have been left with an unmanageable workload," Theresa May, the shadow work and pensions secretary said.
"The tragedy now is that the people who need help the most have been cast adrift by Labour's incompetence."
"Until we have some honesty... about the extent of the problems plaguing the Jobcentre network we won't be able to fix the problem."
But Labour said the Tories had presented a one-sided picture of the report, as it had stated that the delivery of the services improved after initial teething problems common to all new programmes.
"This is utter hypocrisy from the Tories," said employment minister Jim Knight. "They accuse Jobcentres of being unprepared while at the same time opposing the extra £5bn we have invested in them."