WASHINGTON — The Justice Department has apologized to a federal appeals court for providing inaccurate information about a central issue in a case challenging the constitutionality of a disputed law-enforcement power known as national security letters.

The letters are a kind of subpoena that the F.B.I. can issue without court oversight. The case centers on the constitutionality of a gag rule that forbids companies from disclosing whether they have received such letters.

The Justice Department said it had misled the court by incorrectly saying that telecommunications companies were permitted to disclose that they had received at least one such letter seeking records about a customer. In a letter unsealed this week, the department said that the misstatement was “inadvertent.”

It is the latest in a series of inaccurate statements that the executive branch has made to other branches of government about surveillance rules and practices, many of which have come to light during the scrutiny on data collection that came after the leaks last year by the former intelligence contractor Edward J. Snowden.