Within the past month the Navy has cut its number of security clearances by 19 percent and lowered the clearance levels of another 7 percent of those with access to classified information, Navy Secretary John F. Lehman Jr. said yesterday.

Lehman, who last month ordered an immediate 10 percent cut in existing clearances said in an interview on NBC's Today show that his ultimate goal of a 50 percent reduction in clearances was attainable.

L. Britt Snider, the Defense Department's Director of Counterintelligence and Security Policy, said in an interview with The Washington Post last week that he believed the 50 percent goal was unrealistic.

"I don't want to discourage the Navy from trying," Snider said.

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."

Lehman said yesterday that Snider was "unduly pessimistic . . . We know we can do that reach the 50 percent goal ."

Capt. Brent Baker, the Navy's assistant chief of information, said the 50 percent goal would be met in part by reducing some clearances to lower levels as well as by withdrawing other clearances entirely.

Baker said it was not yet clear when the goal could be attained. Lehman had said he wanted the 50 percent reduction "as soon as feasible."

Lehman said that on the aircraft carrier USS Nimitz alone, 64 percent of the clearances had been withdrawn or reduced in level. Accused spy Michael Lance Walker, one of four Navy men whose arrests on espionage charges prompted Lehman's order, served as a seaman aboard the Nimitz, where he had access to the ship's "burn bag" of classified documents to be destroyed.

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

Defense Secretary Caspar W. Weinberger last month announced a 10 percent cut in the 4.3 million Defense department and military contractor employes now holding security clearances, to be implemented by Oct. 1.

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

Although Lehman had ordered an immediate reduction in the number of military contractor employes holding security clearances, a Navy spokesman said yesterday that figures on the number of clearances cut for contractor employes were not yet available.