Her current four-year term runs out on June 30.

The High Commissioner's job, created in 1994, is central to the U.N.'s credibility as a protector of international human rights. Its holder has more or less free rein to comment on situations affecting rights around the globe.

Although Arbour is known to have upset several countries, including the United States and some Arab states as well as Israel with statements on rights issues, diplomats said she was unlikely to have taken her decision because of criticism.

Once chief U.N. war crimes prosecutor and a former Canadian Supreme Court judge, she is understood to be planning to tell the U.N. Human Rights Council that she will leave in the summer when it opens a four-week session next Monday, the sources said.

"She has been thinking about it for a long time and she has decided against continuing for another term," said one source close to the U.N., who declined to be identified.

There was no formal comment from the Arbour's office in Geneva. Her spokesman, Rupert Colville, said: "She has not made her intentions known."

NOTIFIED U.N. CHIEF

The sources said Arbour, 61, had notified U.N. Secretary-General Ban Ki-moon, who has to decide on a successor. Arbour was picked by Ban's predecessor, Kofi Annan, and approved overwhelmingly by the General Assembly in 2004.

The high commissioner reports to the Human Rights Council but is not controlled by it and Arbour has criticised its work. She also directs a gamut of programmes of technical assistance and training on rights protection in countries that seek it.

U.S. officials are known to have been angered recently when she praised Cuba for "positive engagement" on rights issues, and Arab nations were upset when she implicitly criticised their own Charter of Human Rights after initially praising it.

"But that is unlikely to have played a a key role in her decision," said one diplomatic source. "She is pretty tough-minded and firm in her views." Another said she may be considering a political position in Canada.

Arbour, previously best known as the chief U.N. prosecutor trying crimes committed during the Balkan wars in the 1990s and the 1994 genocide in Rwanda, has also been outspoken on the U.S. stance on torture.