The British government claimed today to have shot down as many as 11 attacking Argentine warplanes yesterday, but a British war correspondent on the scene reported that British forces also suffered heavy casualties aboard a ship hit by Argentine bombs.

The casualties came during a major British landing operation near the Falkland Islands capital of Stanley in some of the heaviest fighting of the war, Independent Television News reporter Michael Nicholson said. The Defense Ministry acknowledged that three of its ships, including a frigate, had been damaged in the attacks.

The landing was part of a major British operation to unload men and materiel at new bases at Port Fitzroy and Bluff Cove in preparation for an expected final assault against Argentina's last major stronghold 15 miles northeast at Stanley.

Argentina, in its account of the battle, said its warplanes sank the frigate Plymouth and damaged four troop landing ships, Sir Tristram, Sir Galahad and Sir Belvedere, Washington Post correspondent Margot Hornblower reported from Buenos Aires. The Argentine high command said two Argentine planes were shot down during the attacks.

Argentine military sources portrayed Britain's assault as a failure. In a briefing tonight, military spokesman Enrique de Leon said Argentine troops last night repelled a British attack on Stanley defense lines from positions on Mt. Kent.

"Yesterday's actions were part of a grand effort to take Stanley," De Leon said. The British retreat, he said, showed that "they don't have the strength."

British Prime Minister Margaret Thatcher, in interviews with American television networks today, suggested that it has taken considerable time to move the British forces and their equipment into the best positions for "the final assault."

"It is almost impossible for me to say" when the battle for the Falklands capital would begin, she said. "I hope [it is] not too distant, because all our troops are there in very difficult circumstances . . . .

"But the time when the military commander moves is up to him," Thatcher said. "We can't fight a battle round the Cabinet table. He can only fight it on the spot."

Dispatches from British correspondents on the Falklands continue to blame windy, wet and cold winter weather for slowing the British progress by making helicopter flying hazardous and miring overland travel in deep mud.

Yesterday's battle, one of the more costly in the war for both sides, came when a break in the thick cloud cover over the islands allowed waves of Argentine Mirage and A4 Skyhawk warplanes to attack the British landing operation.

The British Defense Ministry said tonight that five men were injured aboard the frigate Plymouth when it was hit in the air attacks and that "it is, however, feared that casualties from the attacks on Sir Tristram and Sir Galahad were much heavier. Early reports indicate a number killed and injured." The British Press Association, an independent domestic news service, reported that a fourth unnamed ship, a small landing craft, was also hit and that the Galahad may have been sunk.

The British forces at the newly established bases near Fitzroy, who had not been attacked previously from the air, were caught unprepared for the bombing raid by Argentine A4 Skyhawk fighter-bombers yesterday morning, according to British correspondents who witnessed the attack. They said that mobile Rapier antiaircraft missile systems, which had been unloaded from the ships earlier in the morning, were not yet in place around Fitzroy.

Nicholson said that hundreds of troops on the Sir Galahad had to jump overboard to escape explosions and that he saw scores of casualties as sailors and troops on the ship were hit by exploding ammunition.

During later bombing raids, in which another landing ship and a frigate were hit, the Defense Ministry said up to three Argentine planes were shot down and four damaged. When Argentine Mirages made a final attack at dusk, the ministry said, two British Sea Harrier jump jets shot down four of them.

Meanwhile, Western diplomats in Buenos Aires have begun to take seriously Argentina's claim that the British aircraft carrier Invincible is at least temporarily disabled, correspondent Hornblower reported. They point to the mysterious lack of air cover for British troops during yesterday's landing.

One diplomat said the Argentines are certain that they hit the Invincible, but do not know how badly it was damaged.

Thatcher, asked during the American television interviews today whether any chance remained of avoiding a bloody battle for Stanley, said, "I just hope that once the battle has commenced and they've both taken certain positions that both commanders will be able to arrange a surrender which will save many young lives."

Once the Falklands are recaptured, Thatcher said, they would be defended by longer-range British fighters and bombers using a lengthened runway at Stanley airport, by Rapier antiaircraft missiles and by warships and submarines.

"Yes, we shall have to make arrangements to defend it," she told CBS correspondent Tom Fenton. "We haven't lost British blood suddenly to say, 'All right, now we're going to leave these people defenseless.' "

Thatcher repeated plans to give the Falklands eventual self-government. She said the islands could have a prime minister and legislature but still rely on Britain for defense and foreign relations.

"I hope we will be able to get some help," Thatcher told Tom Brokaw of NBC. "Those islands are strategically important as well as being important in democratic and political terms, and I hope everyone will realize that. Some very big oil tankers have to round Cape Horn to get to Alaska. They are to some extent also the gateway to the Antarctic, which will become progressively more important in resource terms to the world as a whole."

But Thatcher reiterated her opposition to any further negotiations with Argentina now that casualties are mounting in the war. "The most important thing is self-determination" for the Falkland Islanders, Thatcher told Peter Jennings of ABC. "After all, that's what the United States stands for."

The islanders had always "totally rejected an Argentine future," Thatcher said. "I do not imagine after an invasion that they would actually want anything to do with the Argentinians as far as sovereignty goes--nothing, nothing at all."

[Meanwhile, the British Defense Ministry said the supertanker Hercules, which was damaged yesterday in what the British said was an Argentine bombing attack, was making its way to a South American port for repairs, Associated Press reported.]