Ruud Lubbers, the former Dutch prime minister, on Sunday resigned as the UN High Commissioner for Refugees over a sexual harassment scandal.
Lubbers' resignation was sent to Secretary-General Kofi Annan, a UN spokesman said.
"The complaint of sexual harassment could not be substantiated," Lubbers wrote in his letter.
"For more than four years I gave all my energy to UNHCR. Now in the middle of a series of problems and with ongoing media pressure you apparently view this differently," he told Annan.
When the allegations first surfaced last year, Annan decided after taking legal advice that he was unable to take action against Lubbers.
But last week, Annan appeared to think again after a British newspaper, the Independent, published details of an investigation conducted by the UN's internal watchdog service.
The inquiry concluded that the allegations "indicated a pattern of sexual harassment" by Lubbers.