UNITED NATIONS — Secretary-General Ban is expected to turn back, without a response, a letter from Taiwan accusing him of violating the rules of the United Nations by denying the island democracy a seat at the world body.

Cranking up Taiwan’s struggle for international recognition, President Chen delivered his latest plea for U.N. membership to the Security Council on Tuesday, which was the final day the monthly rotating presidency of the Council was held by Communist China, which considers Taiwan a renegade province. Dismissing the move as a “petty trick” by an “international troublemaker,” China’s U.N. ambassador, Wang Guangya, immediately returned Mr. Chen’s letter.

The letter was hand-delivered to Mr. Wang by representatives of the Solomon Islands and Botswana. A separate Taiwanese letter was delivered Tuesday to Mr. Ban. It was to be returned as well, U.N. officials said, just as an earlier letter from Mr. Chen was rejected last week.

In his letters, Mr. Chen used several arguments that led to a legal standoff. While China and U.N. officials said they relied on a General Assembly resolution, Taiwan said its arguments were based on the U.N. Charter, the world body’s founding document that is similar in U.N. terms, in its bedrock status, to America’s Constitution.

The episode comes as Beijing, in the run-up to the 2008 Olympics, is facing increased scrutiny of its internal human rights record and its role in the humanitarian crisis in Sudan’s Darfur region. At least one Democratic presidential candidate has suggested a boycott of the Beijing Olympics, and the Republican front-runner, Mayor Giuliani, has a strong record of favoring Taiwan. The Taiwanese strategy appears to be to press the issue of its belonging to the United Nations until it succeeds in breaking into the club, though its efforts have so far been stymied.

Taiwan, which according to Mr. Chen’s letter to Mr. Ban is an “independent sovereign nation,” is starved for international recognition, and its neighboring superpower is as adamant about denying it any hint of independence. In addition to the historic animosity the Communists hold toward the heirs to Chiang Kai-shek’s Nationalists, China may well fear separatist awakening in places like Tibet.

“There is but one China in the world,” Mr. Wang told Chinese reporters at the U.N. yesterday, according to a translation by the state-run Xinhua wire service. “Taiwan is an inalienable part of the Chinese territory, and the government of the People’s Republic of China is the sole legitimate government representing the whole of China.”

This, he added, is a “stand generally upheld by the international community that fully conforms to the principles of the U.N. Charter.”

After being rejected as an observer in the World Health Organization, Taiwan this year attempted to become a full WHO member. Once rejected, it sent a letter to Mr. Ban on July 23, requesting U.N. membership. That plea was also rejected, prompting Tuesday’s follow-up letters.

“The U.N. secretariat does not have the discretion to reject outright the application” of Taiwan, Mr. Chen wrote to Mr. Wang. Taiwanese officials cited Security Council procedural rules regarding new U.N. membership, which instruct any state wishing membership to apply to the Secretary General, who then should “immediately place the application” in front of the council.

U.N. spokeswoman Marie Okabe said yesterday that according to the 1971 General Assembly resolution that created the U.N.’s “one-China policy,” Mr. Chen’s letter “could not be received and was thus returned by the U.N. Office of Legal Affairs.” In its resolution 2758, the assembly decided to “expel forthwith the representatives of Chiang Kai-shek from the place which they unlawfully occupy,” a reference to China’s general assembly seat.

Mr. Chen argued that his membership request had nothing to do with China. Resolution 2758, he wrote to Mr. Ban, “neither grants China the right to represent Taiwan’s 23 million people at the United Nations, nor states that Taiwan is either a part of China or the People’s Republic of China.”

Taiwan’s membership bid has yet to garner wide enthusiasm at the U.N., where some of the world’s worst human rights violators are leading members of the Human Rights Council.

China’s insistence on denying Taiwan any form of recognition has led to rejection of Taiwanese reporters applications for U.N. entry passes. During the annual General Assembly general debate, meanwhile, the Taiwanese government each fall mounts an expensive ad campaign aimed at building its case to join the U.N.

America adheres to its own “one China policy,” which opposes any change to the status quo, but it also abides by the Taiwan Relations Act, which says it is American policy to preserve “extensive, close, and friendly” commercial and cultural ties with the people of Taiwan, and that America will provide Taiwan with arms to defend itself against Communist China.

One by one, Taiwan’s regional neighbors have followed America and cut diplomatic relations with Taipei, even as Taiwan has overhauled its political system to make it more democratic.

The last neighbor to withdraw its ambassador from Taipei, in the early 1990s, was South Korea, where Mr. Ban served as foreign minister until last year.