The UN high commissioner for refugees, Ruud Lubbers, is under investigation for sexual harassment after a member of his staff filed an official complaint.

He denies the allegation. In a statement read to reporters in Geneva, the former Dutch prime minister said there had been "no improper behaviour" on his part.

The case is now being investigated by the UN's office of internal oversight.

According to the New York Times, the complaint was filed by an unidentified female in her 40s who works as an administrative officer in the agency's human resources department and has served for 20 years at the Geneva-based agency.

Mr Lubbers, who is married with three children, said: "It has been brought to my attention by Dileep Nair, the head of the office of internal oversight, that a complaint about sexual harassment has been filed against me by a UNHCR staff member.

"The complaint refers to a formal meeting in my office on December 18 2003. The meeting was attended by five other staff members.

"The complaint was filed on April 27, more than four months after the alleged harassment. In that meeting of last December 18, there was no improper behaviour on my part."

The woman claims that the alleged incident occurred at the end of the meeting as she and other staff members were leaving the room. She later told colleagues that she was "shocked and horrified" by what had happened.

A UNHCR spokesman, Ron Redmond, declined to comment further.

Mr Lubbers, 65, was a Dutch prime minister from 1982 to 1994 and was elected to head the UN's refugee agency in October 2000, succeeding Sadako Ogata of Japan.

His term, extended last year, expires at the end of 2005.

Mr Lubbers was in Washington yesterday to meet the US secretary of state, Colin Powell, and other officials to discuss humanitarian crises in Sudan and elsewhere.

He led an investigation into the sexual exploitation of children in refugee camps in west Africa by local workers hired by the UNHCR. After a five-month inquiry, 10 cases out of 43 were proven, although none involved a UN staff member.

Mr Lubbers told the Associated Press at the time: "There is absolutely no place in the humanitarian world for those who would prey on the most innocent and vulnerable of the world's refugees - the children. There must be zero tolerance."