Navy Secretary John F. Lehman Jr. moved quickly yesterday to dispute the notion that the destruction of a British warship with a single missile means that all navies are now vulnerable and that the Reagan administration should alter plans for a huge expansion of U.S. surface ships and aircraft carriers.

The Navy's top civilian argued just the opposite. He contended that it is the big American aircraft carriers--in contrast to the very much smaller and less capable British carriers now in the South Atlantic--that provide enough planes for a defense that any enemy would have trouble penetrating.

Discussing the implications of the dramatic events around the Falkland Islands with reporters yesterday, Lehman said the missile-firing Argentine jet that knocked out a British destroyer on Tuesday "would not have gotten anywhere near" a U.S. battle fleet without being challenged by missile-firing F14 fighters from big American aircraft carriers.

Without criticizing the British, Lehman also said the U.S. fleet "would not put any ship alone outside the range of air cover" from carriers nearby because small destroyer-type vessels in that situation are especially vulnerable to the kind of attack that Argentina launched.

The United States now has 13 big aircraft carriers and Lehman wants two more, at $3.4 billion apiece, as part of a five-year $96 billion shipbuilding program aimed at 15 carrier battle groups with vast arrays of protective vessels.

The Navy chief argues that these big ships, with 90 or 100 jets aboard, carry enough radar surveillance planes, electronic warfare planes and fighters to keep an aerial hunter-killer force aloft 24 hours a day with the ability to "see" in all directions out to a distance of some 450 miles from the carriers.

The small British carriers Hermes and Invincible are good ships, Lehman said, but they have only about 10 short-range Harrier jump-jets each and can't provide anywhere near the protection over an area offered by U.S. groups. "The Harriers are good but have very limited range and short-range radars and they don't have enough of them to protect the formation all the time," he said.

Lehman calls the British ships "Gary Hart carriers," a reference to Sen. Gary Hart (D-Colo.) who has pushed for more smaller and cheaper carriers to take on some of the less threatening missions rather than the big new nuclear-powered vessels Lehman wants. Yesterday, Hart retorted that he was not proposing British-style jump-jet carriers but smaller versions of the U.S. carriers.

Lehman argues that the Falkland battle shows there are not really any low-threat areas in the world when it comes to high-quality weapons.

The Argentines used a new French-built Super Etendard fighter and French-built Exocet missile, fired from about 20 miles away, to knock out the British destroyer.

The Exocet is in the hands of many countries already. The missile flies just above the waves, making it hard to spot, and has its own radar guidance system, allowing the pilot to get away soon after firing the missile.

Lehman said the Exocet "is a very capable cruise missile," but "we are confident we can handle that through the layered defense" of fighters and screening warships.

The Soviets have even better missiles but he argued also that the big new attack carriers "are designed to absorb" blows from Exocet, which carries a 360-pound explosive warhead, and even from 3,000-pound explosives. New carrier construction and the big damage control crews, he asserted, ensure that even five or six hits with torpedoes would not disable the big ships.

Lehman's pre-emptive strike against those concluding that the so-called smart weapons make surface ships obsolete obscures the fact that there is a shift in thinking within the Pentagon itself about the best way to protect the American Navy and threaten the Soviet one.

Lehman himself even has gone to the point of approaching Air Force leaders about taking over some of the Navy's job of covering the Soviet fleet. His proposal was to use some of the land-based planes of the Air Force to keep Soviet ships under the gun.

The Air Force dramatized the ability of its long-range B52 bombers to fly halfway around the world and attack ships by staging demonstration flights over Soviet ships in the Indian Ocean during the Iranian hostage crisis. Also, Air Force leaders have said that B52s equipped with better aiming devices and armed with antiship missiles would pose a big threat to Soviet vessels.

However, Lehman has said on other occasions that he found the Air Force unreceptive to taking over the antiship role it seemed to covet years ago, perhaps because the service fears it already has too few planes for the jobs at hand.

The Soviet high command has shown no such reluctance to use its long-range bombers to cover American ships. Every other Soviet Backfire bomber coming off the production line is put on naval duty. The Backfire armed with long-range cruise missiles is considered by U.S. naval leaders to pose one of the biggest threats to warships like aircraft carriers and cruisers.

Lehman has acknowledged that the U.S. Navy must pose the same kind of offensive threat to the Soviet fleet. Navy planes armed with Harpoon antiship missiles, which have a range of about 60 miles, are seen as the short-term answer. A longer-range Tomahawk cruise missile for sinking ships is under development.

Defensively, Navy leaders have high hopes they will be able to stop an incoming missile with an outgoing one, or, failing that, could fill the air around the ship with lead to stop any missiles getting through the outer defense.

The threat from smart weapons will get worse before it gets better, according to many analysts, giving the edge to the attacker who can fire highly accurate antiship missiles from aircraft, ships and submarines. New weapons have shifted the advantage back and forth between defender and attacker over the years. Today, smart missiles and torpedoes seem to be doing more for the offense than the defense at sea.