Britain lost two more carrier-based fighter planes in its undeclared war with Argentina over the Falkland Islands, the Ministry of Defense announced today.

The aircraft, Sea Harrier vertical takeoff jets, disappeared from radar screens after taking off on antiaircraft patrol missions, spokesman Ian McDonald said. When the Harrier jump jets failed to return after their fuel capacity would have expired, the pilots were listed as missing and presumed dead.

Loss of the two jets dealt a serious blow to the ability of the British naval task force in the South Atlantic to defend itself against Argentine aircraft, which have numerical superiority.

McDonald gave no indication that the planes were shot down by the Argentines but instead implied that they were victims of a mishap.

He said the ministry did not know the reason for the planes' disappearance, but added, "The weather and visibility were very poor." Search-and-rescue operations are continuing, he said, but next of kin of the pilots have been notified.

Earlier, he had said there had been no reports of any military action for the second consecutive day in the 200-mile blockade zone proclaimed by Britain around the Falklands.

An Argentine military communique also said that there had been no fighting in the Falklands Thursday, Washington Post correspondent Jackson Diehl reported from Buenos Aires. Referring to Britain's report of the loss of the two Harriers, the communique said, "it will be remembered that in previous communiques it has been reported that the Argentine armed forces shot down nine Sea Harriers."

The task force of warships around the Falklands started out with only 20 Harrier jump jets as the main air-defense system for the ships. It has now been reduced to 17 to contend with Argentina's more than 100 American- and French-built jet fighter-bombers.

A Harrier was shot down by Argentine antiaircraft fire Tuesday and the pilot killed during a bombing raid on an airstrip in the Falklands, which were seized by Argentina from Britain five weeks ago.

Today's losses mean that the task force has been deprived of 15 percent of its jet aircraft.

With the loss of each plane it becomes "more difficult" for the task force to guard the ships and prevent Argentine violation of the 200-mile blockade zone Britain has proclaimed, a military analyst noted. He said the two countries were involved in an "air war of attrition."

Twenty more Harriers are en route to the fleet aboard a container ship that left Britain almost two weeks ago and is expected to reach the vicinity of the Falklands shortly.

Those 20 Royal Air Force Harriers, converted for use on the two aircraft carriers in the fleet, are regarded as essential for any invasion of the Falklands Britain may carry out if diplomacy fails to achieve the removal of the 6,000 to 10,000 Argentine troops from the islands.

Argentina demonstrated the vulnerability of the fleet Tuesday when a Super Etendard fighter-bomber avoided detection by radar or Harriers and demolished the British destroyer HMS Sheffield with an Exocet sea-skimming missile.

The Defense Ministry announced today that 20 members of the ship's crew "must now be presumed dead." Five officers, including two lieutenant commanders, were among those killed. Earlier it had been feared that the death toll was 30.

Another 24 sustained injuries, and one man was listed in serious condition. Correspondents on task force ships had reported earlier that most of those injured were suffering burns. The remaining 242 crewmen were well and have been transferred to other ships in the task force, McDonald said.

As a result of the loss of the Sheffield, it is believed that the task force has moved farther east of the Falklands to stay out of range of Argentine planes.

It is becoming increasingly evident that the task force has serious gaps in its defenses for this mission.

The Navy's main function is to guard against attack from the Soviet Union, and for this close-to-home operation it can rely on the Royal Air Force and its reconnaissance planes for early warning.

Since the mid-1960s, when the British military withdrew militarily east of Suez, the fleet has reduced its capability of taking naval action far from home--and the Falklands are 8,000 miles from Britain.

Thus, the overworked Harriers are the main line of defense and detection for the task force in the 200-mile zone, and they have also borne the brunt of carrying out air raids on the Falklands for the last week.

Defense Secretary John Nott acknowledged in Parliament yesterday that the Sheffield was vulnerable because it did not have the new Sea Wolf missiles, the major weapon in the British arsenal to defend against sea-skimming missiles such as the Exocet that fly below radar range. He explained that the Soviets do not have sea-skimming missiles, so the Sea Wolf was not given priority.

Only two ships in the task force are armed with the Sea Wolf, but more missiles are now being rushed into production.

As a result of the Falklands conflict, Britain is also bringing forward the completion dates for an aircraft carrier and a frigate that originally were scheduled to go into use in September.

It was announced today that another 10 Sea King helicopters would be sent to the South Atlantic on a container ship. The task force is believed to have about 35 Sea Kings already. Their main function is antisubmarine warfare or to transport as many as 27 paratroopers for assault missions.

There has been growing concern, since the attack on the Sheffield, that Argentina may try to use its three old, diesel-powered, West German-built submarines to strike at the two aircraft carriers in the task force. Loss of an aircraft carrier, although thought unlikely, could cripple the British mission.

United Press International added from London:

Replacing the Sheffield and the three lost Sea Harriers could cost Britain nearly $380 million, experts said. The destroyer cost $50 million when it was built in 1971, but inflation has pushed the replacement cost well above that.

The Ministry of Defense estimates that a similar destroyer today would cost $180 million, but some observers say inflation and the need for more sophisticated equipment on the ship would push the total to over $360 million.

The lost Sea Harriers will cost between $9 million and $10.8 million each to replace, experts said. Min- istry of Defense sources have said the total value of the fighting ships in the task force is probably around 3.6 billion