Britain announced yesterday a ruling in a century-old border dispute between Argentina and Chile that may have a long-range bearing on the question of who owns parts of Antarctica including what may be valuable petroleum deposits.

The ruling, reached six months ago by an international arbitration panel and ratified yesterday by Britain, awarded Chile three small islands in the Beagle Channel at the southern tip of South America.

Chile immediately accepted the decision, but the Argentina Foreign Ministry hinted that it might not.The two countries have mine months to prepare their positions on the ruling.

Under the ruling, Chile will get a group of penguin-populated islands and, more important, the possibillity of miles out from their shores. These 200 miles not only enhance any future Antartic claim, but encompass what some geologists believe are large undersea oil deposits.

To decide ownership of the islands, Chile and Argentina asked the arbitrators, a panel from the International Court of Justice, to determine the path of the Beagle Channel, an intercoastal waterway that wends from the Atlantic to the Pacific.Under an 1881 treaty, the channel is the dividing line between Chilean and Argentina territory.

At the time the treaty was written, neither country was particulary concerned about the seemingly worthless islands, and it took them several decades to realize that there was some disagreement as to where the Beagle Channel actually was.

The channel was named for the early 1830s expedition of British surveyor Capt. Robert FItzroy and naturalist Charles Darwin aboard the H.M.S. Beagle!

Fitzroy and Darwin subsequently wrote books about the expedition, with Darwin noting plant and animal life and Fitzroy meticulously charting the three islands, Picton, Neuva and Lennox. Fitzroy's logs refer to the distance between the oceans as the Beagle Channel, but do not say just which path - to the north, south or in between the islands - the designated channel took.

During the last half of the 19th century, Chile and Argentina occupied themselves with mainland border disputes to the north of the islands. Starting at the northern end of their border of more than 2,000 miles, they worked their way down the continent, eventually dividing it along the back-bone of the Andes mountain range. In the far south, Chile ended up with the Straits of Magellen, dividing the mainland from Tierra del Fuego island, which was split down middle.

The question of the southern islands was pushed into the background, according to one Argentine Foreign Minstry official, "Primarily because everyone forgot about it."

The 1881 treaty had seemed to settle everything by giving Argentina all the islands to the north and east of the channel and Chile all those to the south. Each country had its own idea of where the channel was, however.

Not until 1967 did the issue become a serious bone of contention. Acting under a later supplement to the 1881 treaty. Chile unilaterally submitted the dispute to the British government for arbitration. Argentina, judging the British biased because of a separate long-running dispute over the British claimed Falkland Islands in the Atlantic, unilaterally refused the arbitration.

In 1971, two countries agreed to submit the issue to the fire-judge panel, whose election would go to the British. [WORD ILLEGIBLE] could accept or reject it, but not [WORD ILLEGIBLE]

For six years, Argentine and Chilean researchers and historians have combed 19th-century sea records and charts, looking evidence to support their [WORD ILLEGIBLE]. Last year, each country took the judicial panel on separate sea [WORD ILLEGIBLE] trips the islands. In an extra effort to end legitimacy to its ownership. Chileas installed a few hundred sheep several settlers on Nueva and Pict islands.

In December, [WORD ILLEGIBLE] six years of oral and written arguments from the two countries, a panel of judges reached a decided and in secret, handed it over to the British.