Pillay is now a judge at the International Criminal Court in The Hague. As a lawyer in South Africa, she defended anti-apartheid activists and championed the right of Nelson Mandela and other dissidents to legal assistance.

Several diplomats, who spoke to Reuters on condition of anonymity because the appointment is not yet official, said U.N. Secretary-General Ban Ki-moon was expected to announce Pillay's appointment as early as Friday.

Arbour, a Canadian, said in March she would not seek a second four-year term as the Geneva-based U.N. High Commissioner for Human Rights after her term expired on June 30.

The daughter of a bus driver, Pillay grew up in a poor Indian neighbourhood in Durban. She was born in 1941 and, as a member of South Africa's Tamil minority, faced discrimination during the apartheid years because of her dark skin.

As a defence lawyer in the early 1970s, she helped expose the use of torture and unlawful methods of interrogation in South Africa.

In the 1980s, Pillay got a second law degree from Harvard University, where she focused on human rights and international and indigenous law. From 1995 she was a member of the International Criminal Tribunal for Rwanda, eventually becoming its president.

Two others on the short list for the job were Hina Jilani of Pakistan, a special U.N. envoy on human rights defenders, and Juan Mendez of Argentina, a special envoy on genocide.

The high commissioner's job is one of the most high-profile and politically controversial in the United Nations. Arbour's views have angered some Islamic and African countries on the Geneva-based U.N. Human Rights Council, although she has also criticized Israeli and U.S. policies.

Some countries on the 47-nation council, which has been accused by Western critics of spending a disproportionate amount of time castigating Israel, would like to have the commissioner answerable to the council rather than to Ban.

Pillay's appointment would be subject to approval by the U.N. General Assembly.