The largest naval task force ordered by Britain into a military confrontation in a quarter century was completing preparations today to sail for the South Atlantic Monday in an effort to force Argentina to relinquish the Falkland Islands, which it seized last week.

The Portsmouth and Plymouth naval bases on England's southern coast were a flurry of activity. Royal Navy seamen and dock hands worked around the clock loading everything from vertical-takeoff jet fighters, missiles, vehicles and ammunition to crates of food, kegs of beer and thousands of bottles of cola onto the aircraft carrier Invincible, the flagship of the British Navy, which will lead the armada of nearly 40 vessels.

The carrier's crewmen, who have never seen combat, were given a last few hours ashore to say goodbyes. Among them was Queen Elizabeth II's second son, Prince Andrew, an attack helicopter pilot. Thousands of tourists crowded into the busy dockyard in the spring sunshine, but they were kept away from the Invincible and other warships.

Before personally inspecting the preparations at Portsmouth this afternoon, Prime Minister Margaret Thatcher's defense secretary, John Nott, declared in a national television appearance, "We are going to restore British administration to the Falkland Islands. That was the prime minister's commitment, and we mean to stick to it, even if we have to fight."

Nott said no military option had been ruled out, from a naval blockade to an all-out assault on the Falklands or even an attack on the Argentine mainland 300 miles from the disputed islands. He refused to comment on whether the nuclear-powered hunter-killer submarine Superb, already in the vicinity, might be used against any of the 15 Argentine warships around the Falklands during the two to three weeks before the British naval task force arrives.

Asked on Britain's commercial television network whether the Thatcher government was prepared to order the sinking of Argentine ships, Nott said, "If necessary, yes."

Asked whether he was prepared to order the storming of the Falklands, now heavily reinforced by Argentine forces, Nott answered, "If it is the only and necessary course, we will have to adopt that course. It is one of the options that at some stage we will have to consider."

Asked whether he thought the British people were ready to accept possibly heavy casualties on either side, he said, "We don't want to destroy human life, but we must retain our right to protect our own British subjects where they are invaded by an aggressor who has been condemned by the United Nations."

Under Article 51 of the United Nations Charter, he said, "We are entitled to fight in defense of our own territory . . . and we are putting ourselves in a position to do just that, and to win."

Nott noted that nearly everyone who spoke during an emergency debate yesterday in Parliament on the Falklands crisis "thought it was correct that we should return British administration to the Falkland Islands.

"We are still seeking a diplomatic solution," he said. "That remains, obviously, our overriding objective. But if we cannot do so, and we have to fight, we will."

Nott added, however, that a diplomatic solution "does not look likely." Argentina's military government has rejected the U.N. Security Council resolution calling for its withdrawal from the Falklands, which it calls the Malvinas. Argentine diplomats have said that they are willing to negotiate with Britain but not about Argentine sovereignty over the islands.

The Thatcher government has committed itself to what Nott repeatedly mentioned today: restoration of British administration of the islands. This could leave room for a diplomatic compromise: allowing Argentina ownership of the Falklands with Britain administering them under a long lease similar to its lease from China of Hong Kong

In U.N.-sponsored negotiations that dragged on for 15 years before last week's invasion, Britain has suggested this option to both the Argentine government and representatives of the Falkland Islands' 1,800 residents of British descent. But both rejected it.

Asked whether the British public, which has appeared much calmer about the crisis than Parliament or the media, would continue to back a military solution, Nott said, "I do not doubt that there will be some people in Parliament who will now withdraw from a belief which they held yesterday and start outlining all the difficulties, outlining all the reasons why we should not fight, outlining all the reasons why we should just give up our territory.

"I can see it developing, but we have made this commitment," he said. "We're not putting the largest task force available in the world--outside the two major super-powers--to sea without the intention, if we have to, of using it."

Leading the task force will be the two-year-old, 19,500-ton carrier Invincible, which the Thatcher government recently decided to sell to Australia next year for budgetary reasons. It and the 28,000-ton carrier Hermes will carry Harrier vertical-takeoff jet fighters and Sea King attack helicopters that can be used for antisubmarine warfare.

The two carriers will sail from Portsmouth Monday with the amphibious assault ship Fearless, carrying a total of 5,000 men. Another assault ship, Intrepid, will sail later. It is expected to carry 1,000 Royal Marines, plus specially trained commando units.

These ships will be joined by frigates from Plymouth before linking up in the Atlantic with half a dozen guided-missile destroyers, more frigates, landing craft and support ships, and possibly two more submarines, that were on maneuvers off Gibraltar and were already ordered to start sailing south.

British paratroopers also have been put on alert, but the Defense Ministry has been silent about them. Britain's nearest airstrip for launching an airborne assault is on Ascension Island in the mid-Atlantic colony of St. Helena, 3,500 miles north of the Falklands, which are more than 7,000 miles from Britain.

Britain's former empire has shrunk since World War II from 58 colonies, including India, to a dozen dots on the map. Britain's last major military intervention abroad, an invasion with France of Egypt in the 1956 Suez Canal crisis, was a fiasco that symbolized its postwar decline as a world power.

In contrast to the nearly unanimous demand for military action on the Falklands during the parliamentary debate and by a majority of the daily national newspapers yesterday, three Sunday weeklies raised reservations today.

The Observer concluded that "probably the best deal that Britain can hope to rescue from this debacle with the help of her friends is a compromise settlement which gives the islanders the best deal available under Argentine rule."

Britain's ousted governor of the Falklands, Rex Hunt, and more than 80 other evacuees left Montevideo, Uruguay, Sunday for London aboard a British Air Force jet. Uruguay had granted the evacuees sanctuary until Britain could bring them home. The group included 79 Marines, the governor, his wife and son and several other civilians, The Associated Press reported from Montevideo.