Throughout the 1982 Falklands war between Britain and Argentina, British intelligence interception of Argentine military radio and even private international telephone transmissions was "impressive, indeed and without it we would never have achieved what we have."

That quote is from an extraordinary diary of a British naval officer who served on the nuclear submarine HMS Conqueror, which on May 2, 1982, sank the Argentine cruiser General Belgrano, taking the lives of about 368 Argentine seamen out of a crew of more than 1,000.

According to the diarist, and in contrast to official statements in London at the time, the Belgrano was sunk on orders from London despite the fact that it was known to be outside the 200-mile exclusion zone established by Britain around the Falklands.

In addition to a gripping account of the sinking of the Belgrano, the handwritten, 51-page diary contains new details on the military and political conduct of the war. Much of it deals with what have become subjects of heated debate in Britain, as long-secret documentation there has begun to leak into parliamentary debate and the public domain, along with charges that the government of Prime Minister Margaret Thatcher intentionally misled the press and public about key aspects of the conflict.

During the Falklands war, which began on April 2, 1982, with Argentina's military occupation of the British islands and ended with the Argentine surrender on June 14, the largest naval battles conducted since World War II took place. Argentina has long claimed the islands, located about 300 miles off its coast.

Throughout the 73-day conflict, five British and as many as 11 Argentine ships, as well as dozens of British and Argentine aircraft, were lost. Britain lost more than 240 men, Argentina more than 1,700.

A copy of the diary was made available to The Washington Post early last month. In numerous passages it describes largely unsuccessful efforts by the Argentines to obtain military assistance from other countries, including Paraguay, Venezuela, Peru and Libya.

In several references, the diary lends authoritative support to reports during the war that Britain received military intelligence information from Argentina's rival and neighbor, Chile. And it indicates that the Argentines indirectly may have been aided by one of their principal trading partners, the Soviet Union.

One series of entries records the presence of Soviet planes and submarines along with Eastern Bloc ships in the Falklands war area. The latter, in a May 24 entry, are described as "a nuisance as they may shield [Argentine] warships closing from that direction. We are all expecting something from there shortly."

The first mention of the Soviet Union came in a May 12 entry with a report on the possibility that Moscow might be trading intelligence with the Argentines because "two Bear [reconnaissance] aircraft have been shadowing the [British] amphibious group." Earlier that day, a lookout on one British Navy ship in that convoy "reported sighting submarine masts today and we therefore suspect the presence of a Soviet SSN [nuclear attack submarine] trailing the group," the diary reports.

Overall, the diary records in a practical way how the well-known capabilities of British electronic intelligence were a key factor in helping London win the war.

The diary, written in a lively, highly personal style, covers the day-to-day activities on the Conqueror from April 6, just after it set sail for the Falklands, until its return to port almost three months later.

British defense officials began seeking all copies of the diary after excerpts from it initially appeared earlier this year in a book published in London, "The Sinking of the Belgrano," by Desmond Rice and Arthur Gavshon.

On Nov. 24, the British Sunday newspaper The Observer published additional portions of the Belgrano passages and, in a Dec. 2 story, identified the diary's author as "former Lieutenant Nyrena Sethia." He retired on schedule from the British Navy in 1982 and is now in the West Indies, "where he set up a yacht chartering business," the paper wrote.

The editor of The Observer, Donald Trelford, was asked by Defense Ministry officials for the diary last month "on the grounds it contained important security information," the paper wrote on Dec. 2. Strict secrecy laws in Britain impose wide-ranging restraints on the press in its coverage of defense matters.

Trelford said the newspaper no longer had its copy and did not disclose its original source.

Attempts by The Post to reach Sethia have been unsuccessful. His mother, contacted by telephone last week, said that she believed he was on the Caribbean nation of St. Lucia but that she had no address or telephone number for him.

Sethia has appeared on one British television show reading excerpts from the diary and has filed suit against the Observer for printing extensive portions without his permission.

Another of the issues under investigation by the British government is the loss of the official navigator's log from the Conqueror, which presumably would contain details of the circumstances under which the Belgrano was ordered sunk. Sethia, through his London lawyers, denies taking any such material, and his suit against the Observer also charges that the paper libeled him by writing that he took "copies of orders and navigational data before he wrote up the diary."

Although it is not clear from the diary what position the writer held aboard the Conqueror, much of the detailed information supports recently revealed public information and previously published books and articles both in the United States and Britain. The Sinking of the Belgrano

The most graphic and controversial passages, published in the Observer, have to do with the Belgrano.

On May 1, according to the diary, the Conqueror discovered the Belgrano and its accompanying vessels while the submarine was patrolling at the edge of the exclusion zone, within which, under the announced British rules of engagement, Argentine ships would be attacked.

The diary describes the trailing of the Argentine warship for a day as it remained out of the established area. The crew was hoping, the diary says, "that our intelligence reports are accurate and that they are going to turn north into the zone."

The May 2 entry, however, begins by describing as "frustrating" the Argentine ships as they "meticulously" sail parallel to the zone "quite unconcernedly."

Then, the diary dramatically describes the Conqueror's receipt of the order to sink the Belgrano "even though it was outside our exclusion zone."

The decision to sink the Belgrano while it was apparently sailing outside the Falklands war area has become a political issue in Britain because Thatcher's defense minister, John Nott, originally announced that the sinking took place as the Belgrano was "closing on elements of our task force, which was only hours away."

The sinking of the Belgrano, and the extensive loss of life in the icy South Atlantic waters, came at a time when the British public was torn between war and a peaceful settlement, which probably would have required some concessions on both sides. The Belgrano sinking, it is now argued by critics of the Thatcher government, guaranteed that there would have to be a military solution.

The diary presents an impressive picture of how the new, costly, computerized electronic intelligence systems now operated by major military powers are integrated into tactical military situations.

"We are evidently able to intercept much, if not all, of the enemy's signal traffic," the diary notes. In various entries, the diary records advance warning of aircraft attacks and messages from top Argentine military officers on the status of their forces.

"The boys in Cheltenham the government's communications headquarters in England where messages are decoded and interpreted know their stuff and have a pretty good idea of what their Argentine intentions are," the entry for May 10 says. Tactical Use of Intelligence

The diary entry of May 7 is an example of the tactical use of intelligence. "Five minutes before the aircraft was sighted today, we received a signal warning us that there was a Neptune antisubmarine aircraft in the area." General alarm was sounded and the crew sent to "action stations" as the submarine dove. "While at the periscope, the coxswain had seen a low-flying aircraft . . . the calculations were that it must have checked us, but probably didn't drop a weapon."

Two weeks before the Argentines on the Falklands surrendered, the diary records that "signals that have been intercepted quote their Falkland Islands naval commander as suggesting that Britain has total maritime supremacy, and their air force is unable to cope with the heavy losses sustained so far."

It goes on to say that Peru "may offer discreet logistics support -- but no more than that, unless the United Kingdom attacks the mainland."

The diary then reports "a telephone call was recently intercepted from the U.S.A. to a Peruvian admiral" that said the officer's son "intimated" some agreement for Peru to provide submarine support to Argentina, a situation that "seems unlikely."

The Argentine government's efforts to gain arms from neighbors was the subject of several bits of information from Chile. On May 28, the diary records a Chilean report that Argentina had approached Venezuela and Paraguay for air-launched Exocet missiles. "Chile is clearly enjoying herself," the diarist wrote, "even if she has been supplying us with unreliable intelligence."

The British radio interception and code-breaking tradition goes back to World War I. In 1917, it was the British who obtained the famous "Zimmermann telegram" -- in which Germany offered to give Mexico the southwestern part of the United States in exchange for military support -- which helped get U.S. opinion behind Britain against the Germans. In the 1940s, under the code word Ultra, the British intercepted some of Hitler's most private communications. 'Indisputable Advantage'

According to the Rice-Gavshon book, an Argentine military analyst, Dr. J.C. Murguizur, wrote last year that, "Setting aside the indisputable advantage provided by satellite reconnaissance, the British intercepted all our radio transmissions, and almost certainly broke our codes." He said he believes the reason the Argentine headquarters on the Falklands was never bombed, was that "its destruction would have deprived the British of its source."

The book says that interception of Argentine messages was carried out from ships in the Royal Navy fleet, Nimrod surveillance aircraft (similar to, but smaller than, American AWACS planes) and a listening station on Ascension Island, more than 4,000 miles from the Falklands.

If a message could not be decoded on the spot, the book said, it was sent to Cheltenham. There, at the center of Britain's worldwide signals intelligence network, teams of specialists worked 24 hours a day, "to monitor, analyze and decode thousands of Argentine operational and other messages." Argentine Messages

The focus today on the dispute over the Belgrano sinking involves two messages from Argentine Navy headquarters to all its ships to return to their home ports. According to the Rice-Gavshon book, those orders were sent at 8:07 p.m., May 1, and 1:19 a.m., May 2, 20 hours and 15 hours before the Conqueror fired its torpedoes.

Although it earlier had been thought that the Belgrano, sailing outside the exclusionary zone, was about to turn north, critics of the Thatcher government argue that British signals intelligence must have picked up those messages ordering the Belgrano to head westward to Argentina. London, they argue, therefore must have known that the Argentine fleet, and the Belgrano, were in fact withdrawing when the order to sink the Belgrano was given.

During this period, as during much of the war, the Conqueror sailed on a defensive picket line, protecting the British amphibious fleet near the Falklands from any attempted attack by the Argentine Navy.

The diarist showed little respect for the Argentine naval forces except for the aircraft carrier The 25th of May and two diesel submarines.

As early as April 30, authorization for the "destruction" of the carrier was received. Whenever it was sighted by British intelligence, however, it was within the 12-mile territorial waters of Argentina -- Nearly 100 miles from even the outer edges of the 200-mile exclusion zone around the islands. Authorization to attack it there was sought by the British fleet, but never received from London.

On April 30, the diary describes passage to an area where the "threats" are from "the cruiser Belgrano -- an ancient, ex-U.S. 2nd World War ship with no sonar or ASW antisubmarine warfare capability, two . . . destroyers Bueno and Bouchard equally decrepit, and an oiler."

On May 2, the diary notes, the Conqueror received authorization to fire at the Belgrano and launched three torpedoes. Two hit the Belgrano. The third hit the destroyer Bouchard, but failed to explode.

Two days after its torpedoes hit the Belgrano, the diary reports that the Conqueror was told by message "we had sunk" the cruiser. Maneuvering in the Night.

During the night, the Conqueror maneuvered behind a ship "on a strange course and speed for a merchantman," only to discover at sunrise it was a hospital ship accompanied by a destroyer.

"We were not authorized to attack the hospital ship or the destroyer, as she was assisting the search for survivors," the diary notes.

On May 6, the diary reports that "intelligence now suggests" the two destroyers that had been accompanying the Belgrano "have bugged off back to harbor -- one with a condenser problem and the other 'to repair storm damage' which is odd since the water has been remarkably clement for the past few days. Perhaps our weapon," it goes on, "which hit but did not detonate did some damage to her." He adds, "One would have thought that a 44-knot MK8 torpedo would have achieved at least something."

On May 7, the British announced that any Argentine ship outside Argentina's 12-mile territorial limit would be attacked. The diary over several days records the anguish at reports of British casualties along with anger and frustration among the Conqueror crew at its inability to get at the Argentine carrier, whose planes were attacking British ships involved in the Falkland invasion.

"The Argentinians seem to be sensibly sitting in port and using their aircraft to weaken us, which they are managing quite successfully," the May 9 entry reads.

By June, however, when the Falkland naval war seemed to have stilled, the Conqueror was permitted to cruise as close as 20 miles from the Argentine coast and then, on June 8, "We have been authorized to pop inside the TML 12- mile limit , just to confirm the presence" of an Argentine Navy group.

"Today's main excitement," it records for June 9, "was returning to the TML this forenoon, and closing to within the six miles of the coast. We must be mad, but, another nuclear first for the Conqueror. Unfortunately we found nothing of interest."

Next: Life aboard the Conqueror