Lockheed Corp. has acknowledged an "inexcusable" and "unacceptable" lapse in security at a California plant after a General Accounting Office investigation revealed that 1,460 classified documents relating to a top secret Air Force program were missing or improperly accounted for.

Congressional sources said the missing documents concern the Air Force's "Stealth" F19 fighter jet, a project so sensitive that the Pentagon has never acknowledged that it exists.

In written testimony prepared for delivery to a House subcommittee today, Lockheed Chairman Lawrence O. Kitchen says there is no evidence of an actual compromise of secret material at the plant. However, Kitchen acknowledges for the first time that Lockheed failed to maintain proper controls over secret documents and prepared "improper" audit forms in an attempt to account for documents that were missing.

"Our internal laxness . . . is inexcusable," Kitchen says in his testimony, a copy of which was submitted by Lockheed to the House subcommittee on oversight and investigations yesterday. "As to secret material, it is clear that administrative controls were not properly enforced."

Kitchen's testimony comes amid controversy about the Pentagon's monitoring of so-called "black" or highly secret programs, which have grown dramatically during the Reagan administration. The Air Force's F19 Stealth fighter jet, which is designed to evade enemy radar, is considered one of the largest and most sensitive of the black programs.

GAO officials yesterday said that during a recent investigation, they identified hundreds of secret and top-secret documents at Lockheed's Burbank, Calif., plant that could not be located by company officials. Also, hundreds of other "discrepancies" were discovered in which documents were not properly audited or their location could not be established for some period of time, officials said.

In one instance, a plant assembler removed classified blueprints showing the configuration of the F19, took them home to his apartment and showed them to his girlfriend, who subsequently contacted Lockheed security officials, according to a subcommittee staff member who has been briefed on the GAO findings.

In an attempt to account for the missing documents, Lockheed officials recently prepared "destruction request" audit forms indicating that many of the missing documents were destroyed years ago. Yet the accuracy of those forms has been challenged by congressional investigators because they were prepared long after the documents were last seen. In his statement, Kitchen acknowledges the preparation of such forms was improper and says he has ordered the practice stopped.

In one instance, a Lockheed official told the GAO he was ordered by a supervisor in April to sign a destruction request form stating he had witnessed the destruction of some missing documents on July 15, 1982 -- more than a year before the official joined the company.

Allegations about the missing documents were first made last month by Rep. John Dingell (D-Mich.), chairman of the House subcommittee. The panel, which will conduct today's hearing, requested the GAO probe after receiving complaints about lax security at the plant from two Lockheed security officials.

In an interview yesterday, one of those security officials, George E. Pointon, said that during an audit in the fall of 1982 he first discovered there were "700 or 800 documents that we couldn't find," a figure he said eventually was brought down to 183 after an intensive search. Yet, Pointon said, company managers never took the problem seriously and severely chastised him for raising the issue.

Pointon also challenged Lockheed's contention that there has been no actual compromise of security on the program. "We know the aerospace industry is heavily targeted by the Soviets," he said. "To say that there is no possibility of a security breach when they don't know where the documents are is a little bit ridiculous."

Defense Department spokesman Glenn Flood yesterday said the security situation at Lockheed "is definitely something we're concerned about," adding that the Air Force is conducting its own investigation. He said he could not comment further, but Donald A. Hicks, the Pentagon's undersecretary of defense for research and engineering, is scheduled to testify at the hearing today.

When Dingell first charged that documents were missing in June, a Lockheed spokesman in Washington dismissed the allegations, saying the company had found no violations of "document-control procedures."

But in his written testimony, Kitchen states there was an "unacceptable looseness" in control over classified material and that he has ordered a "wall-to-wall inventory" of all classified "special-access" documents in the California division.

Kitchen also said the problem developed because senior management failed to conduct required audits and employes were not properly "sensitized" to controls over classified material.

"This is an oversight for which Lockheed management -- up to and including me -- accepts your criticism," Kitchen states.