"She has received a letter informing her of the secretary general (Kofi
Annan)'s decision to summarily dismiss her for misconduct, including sexual harassment, effective immediately," said UN spokesman Stephane Dujarric.
Ms Perelli, head of the UN Electoral Assistance Division, has specifically been accused of "harassment, including sexual harassment, and abuse of authority," the spokesman added.
Charges rejected
Ms Perelli has vowed to fight her dismissal over the sexual harassment charges, which she rejected as false and complained that she was being denied due process.
"I will fight this dismissal and all the charges, which are false," she told AFP from her home shortly after the UN announced that she had lost her job.
Mr Perelli said she would use "every legal route available, internally and externally" to fight her removal.
Mr Dujarric said Ms Perelli would be able to appeal her case before the UN Joint Disciplinary Committee.
He added that her dismissal grew out of an investigation by the UN human resources department that interviewed dozens of people.
"She had many opportunities to respond to those charges. She was given the written allegations of misconduct and was given a chance to respond," Mr Dujarric said.
A separate independent management review of her office produced a report accusing her of favoritism and creating an atmosphere where "sexual innuendo is part of the 'fabric' of the division".
"Zero tolerance (of sexual harassment) means zero tolerance," Mr Dujarric said. "We have a responsibility to act".
The 48-year-old blunt-talking Ms Perelli had won praise for her agency's work in Iraq, most recently during the constitutional referendum in October, as well as in Afghanistan, East Timor and Sierra Leone.
Timing questioned
Before the dismissal's official announcement, US ambassador to the
UN John Bolton questioned the timing of Ms Perelli's firing, less than two weeks before Iraq's legislative polls.
"I think the role of the United Nations in elections is something that we have emphasised, and when the head of the office responsible for the UN's role in the elections is being subjected to press reports about her removal has to lead you to ask the question: What effect it will have on the ground in Iraq," Mr Bolton said.
"So if you are going to make a decision, make it at a time when it doesn't disrupt your programs, why make the decision now?" the US envoy said, noting that she had been under investigation for a year.
But Mr Dujarric stressed that Ms Perelli had not played a significant role in Iraq since October, when Canadian Craig Jenness became the lead UN electoral officer, leading a team of 24 experts.
The Perelli case comes as the UN considers sweeping management reforms required by world leaders at their summit here last September in the wake of the Iraq oil-for-food scandal.
The reforms, strongly pushed by Washington, include giving the secretary general greater powers in exchange for greater accountability, creating a new ethics office and establishing a whistle-blower program to root out corruption.