The International Olympic Committee (IOC) gave formal approval, as expected, yesterday to a plan under which mainland China will be welcomed back into the Olympic Games next year, and Taiwan will be allowed to compete only if it accepts a new Olympic identity, flag and anthem.

The IOC announced at its headquarters in Lausanne, Switzerland, the results of a postal vote in which its members overwhelmingly adopted the solution to the longstanding "two Chinas problem" proposed by its executive board last month and strongly supported by IOC President Lord Killanin.

Under the plan, enacted by a 62-17 vote, athletes from the Peoples Republic of China will be invited to compete in the Olympics for the first time since the 1949 communist takeover of the mainland. They will use their own flag and anthem.

Athletes from Taiwan also will be invited to compete in the 1980 Winter Games at Lake Placid, N.Y., and the Summer Games at Moscow, but only if Taiwanese officials agree to change their name from "the Republic of China Olympic Committee" to "the Chinese Taipei Olympic Committee" and to replace their Nationalist flag and anthem with Olympic substitutes approved by the IOC. The IOC gave the Taiwanese committee until Jan. 1, 1980, to change its name and constitution. Officials in Taipei are expected today to denounce the IOC decision and announce their intention to boycott the 1980 Games, however.

Taiwanese officials had threatened to sue the IOC in the Swiss courts if the proposed "China plan" was adopted, claiming the governing body of the Olympics was breaking its own rules. The Olympic Charter provides that all competing teams must use their national flags and anthems in the opening parade and all medals-presentation ceremonies.

IOC members appear confident that they will prevail in any legal battle with Taiwan, though. In the past two years, Taiwan has lost suits in British high courts which sought to reverse its expulsion from the International Badminton Federation and the International Amateur Athletic Federation on the basis of similar constitutional arguments.

In announcing yesterday's action, IOC Vice President Mohammed Mzali of Tunisia said, "The decision will be immediately applied and there will be no appeal against it." He sidestepped questions about a possible legal challenge from Taiwan, declaring: "The important thing for us is that all athletes may compete in the Olympic Games. We are sportsmen."

Sports officials in Peking are expected to send a nine-member team of speed and figure skaters to Lake Placid and more than 300 athletes in numerous sports to Moscow. Yesterday's IOC action paved the way for invitations to be issued immediately.

Since the Communist regime triumphed in the Chinese civil war 30 years ago, forcing the Nationalist government to the offshore island of Taiwan, no mainland athletes have competed in the Olympics.

A team of 34 was entered in the 1952 Summer Games at Helskinki, but did not take part. The mainland Chinese withdrew from the 1956 Games, and then from the IOC in 1958, to protest recognition and inclusion of Taiwan.

In 1975, China formally requested readmission to the IOC and called for the expulsion of Taiwan. In 1976, the Canadian government refused to admit Taiwanese athletes into the country for the Summer Games at Montreal because the Taiwan Olympic Committee insisted on competing as the "Republic of China."

Lord Killanin has made a strong personal commitment in recent years to bring mainland China back into the Olympics, over opposition of conservative IOC members who maintained that Taiwan had faithfully observed Olympic rules and should not be forced to abandon its flag and anthem.

The pro-Peking movement received a strong and perhaps decisive boost early this year from the United States State Department. It suggested in a letter to the IOC's American representative, Julian K. Roosevelt, that since the United States now recognizes the Peking government as the sole government of China, it would be put in an embarrassing position -- much as Canada was in 1976 -- in Taiwanese athletes came to Lake Placid next February claiming to represent "the Republic of China."