Navy leaders believe the House Armed Services Committee has just fired a torpedo at their program to build only nuclear-powered submarines.

It comes in the form of an amendment to the Defense Department's fiscal 1986 procurement bill on its way to the House floor. The amendment, sponsored by Rep. G. William Whitehurst (R-Va.) and accepted by the panel, 37 to 8, would prohibit the Navy from subsidizing the construction of diesel submarines abroad or obstructing their construction in this country.

If approved by Congress, as appears likely, the amendment would enable U.S. shipyards to build diesel submarines for Israel and South Korea, for starters, and presumably other countries. Navy leaders fear that once U.S. yards start turning out modern diesel boats for foreign nations, Congress will force the Navy to purchase some.

"It's the camel's nose under the tent," said one Navy leader in discussing Whitehurst's amendment. "We want to keep building only nuclear submarines and let our allies build the diesels."

Whitehurst said the Navy has "a deep-seated fear" that a return to building diesels in the United States would result in Congress insisting on adding diesel submarines to the all-nuclear U.S. submarine fleet. He said that was not the intent of his amendment, though he has long advocated such a mix.

One big attraction of diesels is their lower cost. A modern diesel submarine could be built for about $200 million compared with $700 million for the Navy's nuclear attack boats of the SSN-688 Los Angeles class. Another appeal is that diesels are smaller, quieter and harder to detect.

Navy leaders counter that only nuclear-powered boats provide the range, endurance and speed needed for the United States to patrol around the globe. Diesels, they say, are ideal for coastal patrol, a job performed best by U.S. allies.

Navy Secretary John F. Lehman Jr. recently drafted an agreement with Israel that would provide U.S. financing to help build three diesel submarines in Haifa rather than in U.S. yards, which is what the Israelis had proposed. Whitehurst's amendment would negate that deal.

The Senate Armed Services Committee would prohibit the Navy from spending money for shipbuilding until it submits an overdue report on why U.S. shipyards should not be allowed to build diesel boats for foreign nations. The report was due last October.