New British ultimatum

Britain and Argentina steamed closer toward a South Atlantic clash with a new ultimatum April 28 from the British Defense Ministry:

As of 7:00 a.m. (Eastern daylight time) April 30, the British fleet will destroy any enemy ship or aircraft sighted within a 200-mile radius of the disputed Falkland Islands.

However, some political observers claim this latest military challenge to Argentina may shatter the fragile support that Conservative Prime Minister Margaret Thatcher so far has received from opposition parties in Parliament over the Falklands crisis.

The Labour, Liberal, and Social Democrat parties want Mrs. Thatcher to try to negotiate a withdrawal of Argentine troops from the Falklands by diplomatic channels - either through the United Nations or the dogged efforts of US Secretary of State Alexander M. Haig Jr. - before sending in the Queen's Navy to fight the Argentines. Secretary Haig was reportedly prepared to fly to Buenos Aires at time of writing Wednesday afternoon.

This blockade around the Falklands indicates that the British task force is now strategically placed near the conflict zone. A Defense Ministry spokesman said, ''We wouldn't announce this blockade without having the force to back it up.''

However, the Defense Ministry emphasized the siege is scheduled to begin April 30, the possibility of landing a British assault force on the remote eastern half of the Falkland Islands before the deadline is not ruled out. Newspaper reports that a commando force - similar to the one that recaptured South Georgia last weekend - was already ashore in the Falklands were at first denied by the Defense Ministry. However, the ministry has since changed its stance and now refuses to comment on the claim.

The air and sea blockade, which aims to stop the military junta from reinforcing its 10,000-plus force, on the disputed island means that Britain will sink any Argentine vessels moored in Port Stanley harbor and destroy all aircraft - military and civilian - based in the Falklands.

Mrs. Thatcher conferred with her ''inner war cabinet'' Wednesday morning - comprising Foreign Secretary Francis Pym, Home Secretary William Whitelaw, and senior military officials - to discuss what Secretary of State Alexander M. Haig Jr. has described as his ''final set of proposals.'' These were transmitted Tuesday to both No. 10 Downing Street and to President Leopoldo Fortunato Galtieri in Buenos Aires.

Under the latest United States plan, the British and Argentine forces would first withdraw from the Falklands. Then US military units would land on island and stay there during new Argentine-British negotiations.

Three leading Buenos Aires' newspapers have all claimed that the military junta rejected Haig's proposals because they failed to recognize Argentina's right to the islands. However, no official reply has yet been issued by President Galtieri. The Argentine government claims that a British counterinvasion is imminent within the next 48 hours.

With its task force blockading the Falklands, Britain seems in no hurry to reply to Washington's latest peace plan. One government source said, ''It will take several days of study before we reach any conclusion.''

In the House of Commons Tuesday, Mrs. Thatcher rejected calls by Labour Party leader Michael Foot for emergency talks at the United Nations on the Falklands crisis. The British government, she said, had ''taken the view that unless we bring military pressure to bear, the Argentines are unlikely to withdraw.''

Time also weighs against the British. The brutal South Atlantic winter is approaching, unleashing sub-zero temperatures and 40-foot waves, which could severely impair the fleet's fighting capacity.

The Defense Ministry also said that an Argentine prisoner, captured when British Marines retook South Georgia last weekend, died on the island Monday ''in a serious incident.'' The ministry did not outline the exact circumstances of the prisoner's death, but Britain has notified Argentina through Brazilian intermediaries that it is conducting ''a vigorous investigation into the affair.''