The Prince William Board of County Supervisors voted 6-1 on Tuesday to eliminate its audit department, a move that critics say could compromise the independence of the county’s sole internal watchdog.

Instead of having an staff of internal auditors, the county plans to outsource the work to a contractor, which supervisors said would reduce costs and create efficiencies. The vote came over the objections of the county’s auditing department, which warned in a recent memo to the supervisors that if the job is outsourced, “taxpayers will never be able to count on learning the truth about what may be going wrong in the county.”

But Board Chairman Corey A. Stewart (R-At Large) defended the move Tuesday evening, saying that the audit department has been problematic for some time and that it often took too long to submit reports that were sometimes incomplete.

“I understand the frustration some of those folks might have,” Stewart said. “We’re always looking to be better. When something’s not quite up to par . . . we have to change it.”

The county auditors have been told they have 90 days remaining on the job under county policy. One has already taken a job at another firm. Two others said they will begin looking for work.

The auditors strongly disagreed with Stewart’s assessment of their work and said that the problems were not cited until they delivered an audit on Sept. 25 that detailed mismanagement of the volunteer firefighters’ pension program.

“They’ll be rid of all these people and this institutional knowledge,” said Waqar Bajwa, a senior auditor. “They can put somebody in there they can control.”

Audits of county departments will now be done by McGladrey, a national tax and consulting firm that has been working part time for the county.

County Executive Melissa S. Peacor has said that because the contractor would report to the board, it could independently audit county functions, and that there would be no conflicts of interest arising from the new arrangement.

McGladrey has had a long relationship with the county, mostly completing required external audits from fiscal years 2006 to 2010 for a total of $2.6 million. The firm was also given a contract in 2008 for a $30,525 study to look at the internal audit review process. That process was changed in 2009 so that the internal audit department reported to a committee made up of supervisors. The review was also used as a source of information for privatizing the department, according to county spokesman Jason Grant.

The contract with the company is “open-ended,” meaning the contract has no set payment amount.

Audits are approved one by one, Grant said. The Board Audit Committee was made up of Stewart and Supervisors John D. Jenkins (D-Neabsco) and Maureen S. Caddigan (R-Dumfries), with W.S. Covington III (R-Brentsville) serving as the alternate member. On Tuesday, membership on the committee was extended to the rest of the supervisors.

McGladrey has already been retained for two contracts. The first, for $18,500, is intended to let the firm learn how the county’s risk management systems operate and to learn “how risks to the organization have previously been anticipated and maintained.”

The second contract, for $34,600, is for an audit plan for the county, according to the work order provided by the county.

Supervisor Frank J. Principi (D-Woodbridge) was the only supervisor to vote against the plan. Privatizing the job “is not independent in my mind,” he said.