Secretary General of world's worst man-made disaster, the UN, described by aide as "deplorable, but seriously reprehensible"

Wagons were being hastily circled around Ban Ki-moon, the UN Secretary-General, last night as top aides absorbed the shock of one of their own blasting him for allegedly thwarting attempts to combat corruption in the world body and leading it into a "process of decay" and "irrelevance".

The damaging and highly personal charges were made by Inga-Britt Ahlenius, a Swedish auditor who until last week served as the UN undersecretary general of the Office of Internal Oversight Services (OIOS), which is meant to keep the fight against internal fraud and corruption alive. They appeared in an end-of-assignment report to Mr Ban.

Mrs Ahlenius, who spent five years running the OIOS office, accused Mr Ban of attempting to undercut her own authority, notably by seeking to set up a competing investigations unit in his own office and by standing in the way of appointments she wanted to make. And she more broadly questioned his leadership qualities.

"Your actions are not only deplorable, but seriously reprehensible.... Your action is without precedent and in my opinion seriously embarrassing for yourself," Mrs Ahlenius, 72, wrote in an introduction to the 50-page memo, which was first obtained by The Washington Post. "I regret to say that the secretariat now is in a process of decay." She goes on: "Rather than supporting the internal oversight, which is the sign of strong leadership and good governance, you have strived to control it which is to undermine its position." Moreover, under Mr Ban, who was selected in 2008 and whose first term ends next year, the UN Secretariat is "drifting into irrelevance", she contends.

On the face of it, the memo is an excruciating indictment of Mr Ban's performance as manager of the UN body. This will especially infuriate many in Washington. He secured the job with strong support from the Bush White House partly because he promised to bring management rigour to UN headquarters, which at the time was coming out of the scandal in its oil-for-food humanitarian operations in Iraq.