The Defense Ministry today announced Britain's worst losses of the Falklands war, suffered in yesterday's Argentine Air Force attacks: a destroyer sunk by bombs and a merchant vessel knocked out by two deadly French-built missiles. Twenty-four crewmen died, the ministry said.

The losses brought to 100 the number of Britons killed so far in the conflict, which began almost eight weeks ago with Argentina's invasion and seizure of the islands. Argentina has lost at least 370 men, mostly on the cruiser General Belgrano, which was torpedoed by a British submarine.

The Defense Ministry had no information late tonight of any new hostilities today.

[The Argentine military command, which treated the air attack as a major victory, claimed its ground forces on the Falklands Wednesday had shot down two British helicopters and seriously damaged two others, Washington Post correspondent Jackson Diehl reported from Buenos Aires.]

A communique said Argentine Army forces fought with British troops near the British beachhead. It said British casualties had not been determined, but that there had been only one survivor from the two helicopters shot down.

(The military command also said its forces had shot down one of two British Harrier jets that rocketed the Argentine ship Rio Iguzao in the Falklands area. Argentine forces suffered one dead and two wounded, the Joint Chiefs said, without specifying when the attack took place.

Britain said 20 men died and 20 more were injured when the 4,100-ton destroyer Coventry capsized after being hit by 1,000-pound bombs dropped by American-built Skyhawk jets. Also destroyed was the 15,000-ton container ship Atlantic Conveyer, struck by two Exocet sea-skimming missiles fired by two Super Etendard jets.

The attack on the Atlantic Conveyer, which carried war supplies, marked Argentina's first use of the deadly Etendard-Exocet combination since it was introduced three weeks ago and sank the destroyer Sheffield.

The Atlantic Conveyer was the first casualty for the British merchant navy in the task force of more than 100 vessels. Four men were killed, including the first two known civilian casualties in the war, and a "small number" injured, the British Defense Ministry reported.

A frigate, the Broadsword, was also slightly damaged, but no casualties were reported. A frigate is smaller than a destroyer.

A Defense Ministry source said yesterday's attack, launched by Argentina on its independence day, was carefully planned but may have been designed to sink the aircraft carrier Hermes, flagship of the task force with a complement of at least 1,350 men.

Undeterred by the setbacks, Prime Minister Margaret Thatcher refused to relent in the war effort. Dressed in black, she told a Conservative Party women's conference, "Despite these grievous losses, neither our confidence nor our resolve is weakened."

She said the 5,000-man force that Britain landed at San Carlos Bay on East Falkland Island last Friday is "now established with all the necessary supplies," and added that "their spirits are high."

The news of the latest losses for the British task force coincided with the arrival here of the first film footage of war taken by British television. The Royal Navy ships had not previously been able to send film or videotape by satellite, so all movies of the fighting seen here had come from Argentine sources, causing sharp criticism of the Navy in Britain.

The film was dominated by a long interview with Capt. James (Sam) Salt, skipper of the Sheffield, who clinically described how the ship was hit and destroyed May 4.

Even though the film, which came via Ascension Island, was three weeks old, the impact of bringing the war into people's homes via television for the first time will probably be enhanced by the coincidence of today's news with the second deadly use of the Exocet missile.

Alistair Burnett, anchorman of the top-rated evening newscast on commercial television, noted in an unusual personal commentary that the graphic scenes could test the nation's morale and its "willpower," to which he credited the success of the task force so far.

After announcing the losses to a hushed, subdued Parliament, Defense Minister John Nott said: "Our forces on the ground are now poised to begin their thrust upon Port Stanley," the capital of the Falklands about 60 miles east of San Carlos Bay.

"Behind them are another 3,000 men of 5 Brigade," Nott said, a reference to reinforcements aboard the Queen Elizabeth 2, which is believed to be nearing the war zone. A senior Defense Ministry source said the troops would probably be transferred from the converted luxury liner to warships or directly to a beachhead in a few days.

"In spite of the loss of four naval warships," Nott told Parliament, "the task force has more escort vessels today than a week ago. Ten more destroyers and frigates have joined the force in the past two days."

He added, "Generally the military objective to repossess the Falkland Islands has gone forward exactly as we planned it."

His remarks seemed to be part of an effort to maintain a firm course and forestall the possibility of loss of public support as the death toll mounts. It is estimated that another 120 men have been injured, some seriously.

Seventy of the deaths have resulted from the loss of five ships, including two frigates, and another 21 men were killed in the crash of a Sea King transport helicopter.

Nott's delay in announcing the latest losses caused criticism and consternation among many of the families of the more than 26,000 men aboard the task force ships.

The first information came when he was interviewed on the news late last night. At least one ship was "badly damaged and in difficulty," he said. Defense Ministry spokesman Ian McDonald said there was "bad news," but neither gave any details.

McDonald named the two ships 14 hours later, and after almost four more hours Nott told a relieved Parliament of the relatively moderate casualty toll. For several hours there had been fears that most of the 270-men crew of the Coventry had perished.

Many families had sleepless nights until today's announcements. Hundreds of relatives flooded the switchboard with calls at the Navy's crisis center at Portsmouth. A number complained at a meeting with Conservative Party parliamentarian Peter Viggers, who said, "I think a great deal of anguish could be spared if the Ministry of Defense would reveal the names of ships earlier rather than later."

The Defense Ministry has followed a policy, supposedly for security reasons, of not identifying a ship in distress until it is abandoned or has sunk, and then the next of kin are informed.

"The effect on the families of other servicemen is catastrophic in terms of anxiety," an editorial in the Portsmouth Evening News said.

Nott acknowledged the mistake in Parliament today, saying, "I think, in retrospect, we probably should have given the name of the Coventry earlier last night."

The ship, which only went into operation four years ago, was named after the city of Coventry whose famed cathedral was destroyed by German bombers in World War II. It carried a cross of nails made from the charred ruins of the cathedral.

Nott credited the ship with downing most of the five Argentine Skyhawks destroyed yesterday before being sunk by another wave of Skyhawks dropping 1,000-pound bombs. Previously, Britain had reported that only three Skyhawks had been shot down in the action.

The air raids yesterday differed from the four previous days, when the Argentine Air Force staged wave after wave of attacks in an attempt to prevent the resupply of the bridgehead and lost more than 30 planes, according to British claims.

Biritsh military sources claim Argentina has lost more than 50 fixed-wing planes since aerial combat began May 1. Military analysts estimate that the country started out with a maximum of about 150 operational aircraft.

The two ships destroyed yesterday were both north of Falkland Sound and away from the beachhead. A military analyst said the Argentines may have decided it was too costly to attack the heavily defended beachhead since no ship in the anchorage area has been sunk.

Sea Harrier jets off the two aircraft carriers provide the first line of defense, with frigates and destroyers forming a picket line closer in. Landbased Rapier and shoulder-fired Blowpipe missiles are the final line of defense.

The Defense Ministry refused to disclose what supplies the Atlantic Conveyer was carrying except to say that the 20 Harrier jets it transported had already been removed. Some are probably land-based now. It was believed that some helicopters might have been aboard and lost.

The ship was used as a "garage" for the Harriers when not in use because the two carriers did not have room for them. To fly missions, the jump jets would take off vertically, land on a carrier and be fueled and loaded with bombs and missiles before going into action.

The Super Etendard, which was probably refueled in flight, is difficult for Britain's limited antiaircraft defense to detect because it can fly below radar range. In addition, the Exocet flies only 50 feet above the water and has its own guidance system.

At the beginning of the war, it was believed that Argentina had only five of the planes and five of the missiles.

Four Exocets have now been fired to destroy the Sheffield and the Atlantic Conveyer, but it is understood that Argentina is seeking to buy more missiles.

"Theoretically, there is only one Exocet left," a senior Defense Ministry source said. "But I'm a great skeptic. No prudent military commander would operate on that belief.