Victor Korchnoi resigned game 18 of the world chess championship match by telephone yesterday in Merano, Italy, without returning to the playing hall where he faced a hopeless position against defending champion Anatoly Karpov. His resignation gave Karpov the sixth point needed to hold his title, and officially ended the shortest match for the world championship since it came under the jurisdiction of the International Chess Federation (FIDE) after World War II. The previous Korchnoi-Karpov match, three years ago in the Philippines, had been the longest in FIDE history, lasting 93 days and taking 32 games before either player could score six points.

Karpov wins a prize of 500,000 Swiss francs (about $280,000) for the successful defense of his title. Korchnoi's purse is 300,000 francs (about $170,000). Karpov's title cannot be contested again for three years, during which an elaborate, worldwide series of elimination contests will be held to find a new challenger, but there have been rumors that the champion will try to arrange a match before then with ex-champion Bobby Fischer. Karpov is the first world champion to defend his title successfully twice since Mikhail Botvinnik in 1951 and 1954.

The 6-to-2 loss was the most crushing defeat suffered by Korchnoi in the three matches he has played with the world championship at stake. Both previous matches, in 1974 and 1978, were won by a single point. During the 1978 match in the Philippines, Korchnoi predicted the general course of this year's with remarkable accuracy. "It is not easy to play such a match," he said. "In three years' time, I think, my match for the title will not be interesting. But I will fight, as I am fighting now."

Korchnoi will undoubtedly remain active for years at the top level of international chess, but at 50 he is unlikely to have another crack at the world championship. His name will be remembered with those of others who played at a world-championship level but, because of circumstances, never won the title: names like Aron Nimzovich, Akiba Rubinstein, Siegbert Tarrasch, David Bronstein and Paul Keres, which are among the greatest in the history of the game.

For more than five years, since his defection from the Soviet Union, Korchnoi's status as top contender for the world championship has given him a special kind of leverage against the Soviet government. At first, Soviet players refused to participate in any chess event that included Korchnoi, but that policy was changed when it became obvious that it would mean forfeiting the world championship. Now such a boycott could be imposed with little practical difficulty, but it remains to be seen whether the Soviets will do so. It also remains to be seen whether they will allow Korchnoi's wife and son to join him in exile, now that there is no longer any psychological advantage to be gained by keeping his wife in Russia and his son in a prison camp.

Neither player spoke to the press immediately after the end of the match, but the Tass news agency reported that Karpov sent a telegram to Soviet President Leonid I. Brezhnev thanking him for his support and saying he was happy that "your wish has been fulfilled."