The United Nations Commissioner for Human Rights, Mary Robinson, says she will not seek a second term when her four-year tenure ends in September. She made the announcement at the annual six-week session of the Commission in Geneva.
Speaking to reporters after her resignation speech on the first day of the session, Robinson said she was going because resources in her department were so meagre that they had become a major obstacle to her work.
She told delegates she believed she could do more to promote human rights "outside the constraints" of the UN body.
Her decision took many diplomats by surprise because most UN agency bosses tend to seek a second term.
She said that the main "constraints" were financial because her office had to fulfil a huge mandate but only received funding of $21 million, less than two per cent of the UN's regular budget.
She said this meant that her 250 field staff, who monitor human rights abuses, were forced to work on "short-term insecure" contracts; a situation which she found "deeply troubling".
The UN secretary general, Kofi Annan, said he accepted Robinson's decision "with great regret", adding that she had "made a critical contribution to giving human rights a central role in the UN system".
In a statement from New York, he wrote: "She has demonstrated admirable courage in speaking up for those who are voiceless."
Robinson, a former Irish president, took over at the UN agency in September 1997, and quickly gained a reputation for her outspoken attitude towards human rights abuses.
In her speech, she said she had "never felt constrained from speaking out" because she has "always recognised... the importance of standing up to bullies, addressing short-comings and being outspoken".
Diplomats said her remarks were a reference to her having ruffled feathers in major capitals, including Beijing and Moscow, and supported UN investigations into massacres in East Timor and Algeria.
Reacting to Robinson's decision, Russia's ambassador to the UN, Sergei Lavrov, told a news conference: "We didn't agree on everything. We believe she was not entirely objective on some issues.
"I am sure she is an honest person but sometimes emotions drive you beyond the point of objectivity," he added.
Robinson, who will leave office after a global conference on racism in Durban, South Africa, said she was giving notice of her departure because she wanted "to ensure that my successor can be appointed in ample time for a smooth transition".
Her address to the 53-member state Human Rights Commission took place as about 1,000 members of China's Falun Gong spiritual movement held a demonstration outside the UN headquarters in Geneva to protest at their treatment by the Chinese authorities.
swissinfo with agencies