The Justice Department’s internal watchdog said Thursday that his independence has been undermined by the department’s refusal to let him see information derived from wiretaps or national security letters without special permission.

The department’s Office of Legal Counsel issued a 68-page opinion Thursday saying that Inspector General Michael E. Horowitz’s office should not be granted access to several different kinds of typically confidential material unless there is a clear law-enforcement or counterintelligence purpose — and that the department’s lawyers, not the inspector general’s, would make that determination.

That would appear to rule out many of the typical goals of oversight, such as rooting out fraud, incompetence, rule-breaking and cover-ups.

“Today’s opinion by the OLC undermines the OIG’s independence, which is a hallmark of the Inspector General system and is essential to carrying out the OIG’s oversight responsibilities,” said a statement from Horowitz’s office.

Having to get the department’s permission to access files puts “the agency over which the OIG conducts oversight in the position of deciding whether to give the OIG access to the information necessary to conduct that oversight,” the statement said.

The Office of Legal Counsel, in its memo, reasoned that while there are some times when the inspector general’s office should have access to the materials, there are others when it shouldn’t.

Wiretap law, consumer information disclosure policies and grand-jury secrecy rules preclude the release of information for investigations “that are only tangentially related to criminal law enforcement or foreign counterintelligence efforts,” such as “routine financial audits,” the memo said.

Horowitz has been in an ongoing battle with Justice and the FBI over access to such information. He thought he had won when Congress, in passing the department’s 2015 appropriation late last year, included a section intended to improve OIG’s access.

The Office of Legal Counsel shook it off, saying that language was not “a clear statement of Congress’s intent to override [statutory] limitations on disclosure.”