Ruud Lubbers has said the claims are unsubstantiated

In a letter to Secretary General Kofi Annan, Mr Lubbers denied the allegations and said he was quitting because of a lack of confidence in him.

Mr Annan accepted the resignation, thanked Mr Lubbers for his work, and said a new page had to be turned.

The allegations against Mr Lubbers were contained in a confidential UN report leaked by the press on Friday.

It said that the former Dutch prime minister had engaged in unwanted physical contact with a female employee at his agency in Geneva.

The report added that claims made by other employees indicated a pattern of sexual harassment - although these women had not made official complaints.

The document also accused the UN high commissioner for refugees of abusing his authority in trying to influence the outcome of the investigation.

Insult to injury

In his resignation letter, Mr Lubbers, 65, wrote: "The complaint of sexual harassment could not be substantiated."

I ushered the lady out of the room with my hand on her back. And that was all. Ruud Lubbers, UNHCR chief

The high commissioner said he had given all his energy to the agency.

"Despite all my loyalty, now insult has been added to injury," he added.

The claims that Mr Lubbers has sexually harassed a female employee first surfaced last year. But Mr Annan - after consulting lawyers - decided they were not backed up by evidence.

The case re-emerged on Friday, when Britain's Independent newspaper published details of the UN confidential report.

At a news conference following a meeting with Mr Annan on Friday, the high commissioner denied the allegations.

"I ushered the lady out of the room with my hand on her back. And that was all," Mr Lubbers said.

"You could call it familiar but certainly not sexual harassment."

The resignation comes as the UN is already under fire following claims of corruption in its oil-for-food programme in Iraq, and of sexual abuse by peacekeepers in the Congo, our correspondent adds.