The Navy's plan to cut in half "as soon as feasible" the number of people holding security clearances is not realistic, the Defense Department's director of security policy said in an interview yesterday.

"I don't want to discourage the Navy from trying," said L. Britt Snider, Defense Department director of counterintelligence and security policy. But, he said, "It's hard to see how there could be that many people in the Navy that have clearances that don't need clearance."

Navy Secretary John F. Lehman Jr. announced an immediate 10 percent cut last month in the number of Navy and contractor personnel holding security clearances and said he wanted to cut the number of clearances by 50 percent "as soon as feasible."

Lehman said through a spokesman yesterday that he believes the 50 percent goal can be achieved. "It'll be hard, but he believes we can reach it," said Capt. Jim Finkelstein.

Of 1.01 million officers, enlisted personnel and civilian employes in the Navy, 900,000 held some level of security clearance at the time of Lehman's announcement. An additional 1.2 million defense contractor employes were authorized to see classified information.

Lehman's actions were part of a series of measures designed to improve security after the arrests of John Anthony Walker Jr. and three other Navy men on espionage charges.

Cutting the number of those with security clearances is aimed at both reducing the potential sources of information to the Soviets and freeing investigators to do more thorough and timely background checks.

Snider said it would be mere "speculation" to pick a more realistic figure for the number of security clearances that could be eliminated.

Defense Secretary Caspar W. Weinberger last month announced a 10 percent cut in the 4.3 million Defense Department and military contractor personnel now holding security clearances, to be implemented by Oct. 1, and said that the department would consider whether sharper cuts could be made, particularly in the number of people with higher levels of clearance.

At the same time, Weinberger ordered a "substantial reduction " in the number of classified documents.

Snider said yesterday that "the word we get from the services is that they will meet the 10 percent reduction without any problem."

He said, however, that even with a reduction in the amount of information that is classified it will be difficult to make cuts as deep as 50 percent in the number of personnel cleared to see the information. "The two don't relate that closely," Snider said. "Just because you may classify nine documents instead of 10 doesn't mean that the 10 people that are cleared aren't going to need access to one of the documents."

Sen. William V. Roth Jr. (R-Del.), who chairs a Senate Governmental Affairs subcommittee that recommended a 50 percent cut in both the number of military and civilian employes holding security clearances and the amount of classified information, said yesterday he was "fully persuaded that half of those people don't need those classifications."

"I appreciate John Lehman's aggressiveness," Roth said. "There's nothing more important than a substantial reduction in the number of those that have classifications. Everybody wants to be cleared for prestige purposes."

He said that many of those holding security clearances need to see classified material infrequently, and "you could set up special procedures for that. I really think we're going to have to make an aggressive effort or we're going to end up doing nothing. The instinctive push is to overclassify and overclear."

But Senate Intelligence Committee Vice Chairman Patrick J. Leahy (D-Vt.) said he doubted that a 50 percent cut in the number of personnel with security clearances can be achieved, at least in the "short term" of the next five years.

"No one would be happier than myself to be proven wrong on that," Leahy said. But, he said, "You're going to have to redo our whole system of classifying material before you're going to get anywhere close to it . . . . I've seen a photograph from Time magazine marked 'Top Secret.' "