Britain declared tonight that any Argentine ship found within 200 nautical miles of the Falkland Islands after midnight Easter Sunday risks being attacked by British naval forces.

In a dramatic announcement to Parliament hours before U.S. Secretary of State Alexander M. Haig Jr. was due to arrive here seeking a formula for a peaceful Argentine withdrawal from the disputed islands, Defense Secretary John Nott said, "Our first naval action will be intended to deny the Argentine forces on the Falklands the means to reinforce and resupply from the mainland."

While much of the British fleet that left port here Monday is still two weeks away from the Falklands, several high-speed, nuclear-powered British hunter-killer submarines dispatched as a precaution before last week's invasion reportedly have reached the South Atlantic. The 4,500-ton Swiftsure-class submarines are armed with Tigerfish torpedoes.

Nott did not mention the submarines tonight in a list of nearly 30 military and requisitioned civilian ships he named as among those in the task force, and the Defense Ministry has not commented on the subs' location. But former defense secretary Denis Healy, now the foreign affairs spokesman for the opposition Labor Party, said only two of the submarines would be enough to patrol the waters around the Falklands. "No doubt it will be a deterrent to the Argentine Navy," Healy said, "although they will still be able to supply their troops by air."

Since seizing the islands last Friday, Argentina has added thousands of men and considerable military hardware, including tanks, armored personnel carriers and artillery, to the original invasion force. Before Nott's announcement at the end of a seven-hour special debate on the Falklands crisis, several members of Parliament expressed concern about the Argentine military buildup.

Nott told Parliament that Britain was declaring a maritime exclusion zone covering a 200-mile radius from the center of the Falklands from 04:00 Greenwich Mean Time on Monday, April 12--midnight in the Falklands. "Any Argentine warships and any Argentine naval auxiliaries found within this zone," Nott said, "will be treated as hostile and are liable to be attacked by British forces."

Government sources said this action did not require a declaration of war, and Nott emphasized that it would not prevent Britain from taking "whatever additional measures may be needed in exercise of its right of self-defense, under article 51 of the United Nations Charter."

The announcement surprised many members of Parliament, who had earlier been focusing on embryonic efforts to find a diplomatic solution. Welcoming Haig's visit here Thursday, officials said Prime Minister Margaret Thatcher's government remained open to a negotiated settlement on the islands' future if Argentina first withdraws its occupation force in compliance with last week's U.N. Security Council resolution.

Haig's talks here and later in Buenos Aires are seen as crucial to efforts to persuade Argentina to withdraw so negotiations could begin. Diplomatic sources here said Haig is seeking some prospect of a compromise of Argentina's claim to the islands, which the British have ruled since 1833, that would be politically acceptable to both Britain and Argentina.

The most widely discussed option remains the suggestion by British diplomats, made some time ago in negotiations broken off by Argentina before it invaded the Falklands, that Argentina be offered sovereignty over the islands while Britain continued to administer them under a lease, with the islands' 1,800 residents retaining their British identity. This would be similar to Britain's lease of the crown colony of Hong Kong from China.

Argentina neither accepted nor rejected this proposal in the past, but it was strongly opposed by elected representatives of the Falkland islanders and their lobby here. Thatcher and her new foreign secretary, Francis Pym, emphasized in today's parliamentary debate that any settlement would have to be acceptable to the islands' residents.

Thatcher's government also emphasized in public statements today, as it has in diplomatic contacts with the United States and other countries this week, that it will not negotiate until after the military government of Argentine President Leopoldo Galtieri obeys the U.N. resolution and withdraws its forces from the Falklands. "There cannot be any talks or any negotiations or anything of that sort until there is a withdrawal of Argentine troops," Pym said today.

Thatcher and Pym will be talking to Haig, British officials said, only about how the United States can help Britain in its international diplomatic campaign to force an Argentine withdrawal before the British fleet arrives in the South Atlantic in about two weeks.

Well-placed sources here acknowledged that both Haig and the British also will be exploring ways in which Galtieri could be helped to save face while ordering a withdrawal. Besides the lease-back proposal, other options being discussed here include shared owership of the islands or any resources, particularly oil, that may lie under the sea around them.

Under close questioning in Parliament today, neither Thatcher nor Pym would indicate what kind of negotiated settlement would now be acceptable to the British government. When pressed on this by Labor's ex-prime minister, James Callaghan, Thatcher said, "It is the Falkland islanders' wishes that are paramount."

Asked by Callaghan whether the government was seeking to regain British "sovereignty" or only "British administration," which Callaghan said could leave room for negotiating a lease-back arrangement with Argentina, Thatcher said, "I regard the Falkland Islands as still British . . . an invader of unprovoked aggression does not and has not altered the law of British sovereignty."

"It does not matter what we want or what the Argentinians want," Nott said later in the debate, "but what the islanders want. It is their rights that have been taken away by naked aggression and it is their rights that shall be restored."

Diplomatic sources here suggested all this still leaves room for negotiating flexibility, although the Falklands residents would have to be consulted after an Argentine withdrawal if Thatcher's government did not move away from this position. Healy told Parliament the islanders might now be more amenable to a compromise settlement, but few other politicians or officials have expressed similar optimism.

A growing number of members of Parliament are now cautioning against the dangers of military conflict while supporting the Thatcher government policy of at least threatening force. Healy warned the government it had "to tread a narrow path between two dangers," which he said were "selling the Falkland islanders down the river" in a negotiated settlement and "a large-scale military conflict in circumstances which cost us the support of the United Nations and world opinion."

"A forced landing," said Healy, who later approved of the naval quarantine announced by Nott, "would inflict intolerable casualties on the Falkland islanders themselves, whom it is our duty to protect. They are not asking for the peace of a cemetery."

In his first public statement on the crisis since replacing Lord Carrington, who resigned Monday over the government's failure to avert last week's Argentine invasion of the Falklands, Pym told Parliament, "we shall spare no effort to reach a peaceful solution."

But, in a well-received speech intended to rally support for the government, and which was loudly cheered for its tough rhetoric about "unprincipled opportunism of a morally bankrupt regime" in Buenos Aires, Pym emphasized that the threat of military force and international economic sanctions "should show the Argentine regime quite clearly that we mean business."

He said all countries friendly to Britain were being asked to "take measures against imports from Argentina," "follow us in terminating export credits," "encourage their banks to make no more loans to Argentina," and stop "the supply of arms and military equipment to Argentina."

He noted that West Germany, France and Belgium already have banned arms shipments to Argentina. This stops delivery of aircraft, antiaircraft missiles and spare parts and ammunition from France and four warships and a submarine ordered from West Germany.

Senior diplomats from European Community countries are to meet in Brussels on Friday to discuss Britain's formal request for economic sanctions against Argentina. The community imported $1.8 billion in Argentine goods in 1980.

"If the world does not oblige Argentina to restore the Falkland islanders' rights," Pym said, "tomorrow it will be someone else's turn to suffer aggression and occupation."

Later, Nott acknowledged that the Falklands crisis will affect Britain's NATO defense commitments, which increase the burden on the U. S. Navy in the North Atlantic. "While the task force is deployed," he said, "clearly others of our friends will have to fill the gap left by our activities in that area."

"We have no wish to shed blood," Nott said, "but we shall not acquiesce to any act of unprovoked aggression, undertaken presumably in the false belief that we lack courage and the will to respond.

"Let the world be under no illusion. The Falkland islanders are British and we mean to defend them. We are earnest and no one should doubt our resolve."