Mrs Robinson condemned rights violations in Chechnya

Mrs Robinson and Mr Annan are currently in Nairobi, Kenya, attending a meeting of heads of UN agencies, and were not immediately available for comment.

But Joanna Weschler, UN representative for Human Rights Watch, noted that Mr Annan is currently campaigning for a second five-year term, so delaying Robinson's departure would give him the time he needs to find a new human rights commissioner.

"To have two positions of such a high calibre in transition at the same time is obviously quite difficult," she said.

Robinson biography

21 May 1944: Born in County Mayo, Ireland 1969: Appointed Reid Professor of Constitutional and Criminal Law - Trinity College, Dublin 1969-89: Member of the Irish Upper House of Parliament 1988: Co-founds the Irish Centre for European Law 1990: First woman to become President of Ireland 1997: Appointed UN High Commissioner for Human Rights

Mrs Robinson, a former president of Ireland, has been a critic of human rights violations, most notably securing the censure of Russia over abuses in the breakaway republic of Chechnya.

Human rights campaigners described Mrs Robinson as an outspoken crusader - she personally visited 60 countries, including dangerous areas such as Sierra Leone, Kosovo and East Timor.

She said when announcing her decision to quit: "I believe that I can achieve more outside the constraints that a multilateral organisation inevitably imposes".

She also complained that her office receives only about $20m from the UN's $1bn annual budget - not enough to cover the increasing workload of the body that investigates human rights abuses worldwide.

Mr Annan expressed "great regret" when Mrs Robinson announced her decision to quit, saying she had been "a staunch and fearless spokesperson for human rights throughout the world".