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.

Mr Ban was co-chairing a conference on the future of Afghanistan in Kabul yesterday but was due back at headquarters this evening for a meeting with the British Prime Minister, David Cameron. His chief aide, Vijay Nambiar, issued a statement repudiating the attacks by Mrs Ahlenius. "It is regrettable to note that many pertinent facts were overlooked or misrepresented" in her memo, he said.

Privately, some senior UN officials expressed shock at the vitriolic tone of Mrs Ahlenius's memo and wondered if it should be taken seriously. "She has been something of a lone ranger", said one UN official who until recently worked in the UN's oversight department. "It's one thing to be independent, which I think people recognise to be a fundamental of leading an oversight office, it's another thing to be just wilful."

One of her grievances springs from Mr Ban blocking her hiring two years ago Robert Appleton, to head up one of her departments. Mr Appleton had proved effective as head of a temporary task force specifically rooting out UN corruption, the funding for which had been discontinued by the General Assembly. Mr Ban's office resisted appointing him to a new job because of concerns about hiring more women.

That post has remained empty ever since. Nor is there any sign of Mr Ban hiring a replacement for Mrs Ahlenius herself. The leadership vacuum is certain to deepen concern in Washington that Mr Ban is precisely neglecting the area the US was so anxious for him to emphasise. That in turn could jeopardise his chances of re-selection when his term expires in December 2011.

The anxiety is already being expressed loudly by the US mission to the UN in New York. "The United States has consistently and aggressively pushed for a strong and independent OIOS to uncover fraud, waste and mismanagement at the UN, but we are disappointed with the recent performance of its investigations division," Patrick Ventrell, its spokesman, said earlier this month.

Poisoned pen

Extracts from Inga-Britt Ahlenius's 50-page memo to Ban Ki-moon:

* Is the United Nations now on the right path, more transparent, more accountable?... In spite of all your good intentions pronounced in these respects, my answer to this question is regrettably: No

* There is no transparency, there is lack of accountability. 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. I do not see any signs of reform in the organisation.

* I regret to say that the Secretariat now is in a process of decay. It is not only falling apart into silos – the Secretariat is drifting, to use the words of one of my senior colleagues. It is drifting into irrelevance.

* The weakening of the Secretariat and its position in the eyes of Member States also translates into a weakening of the overall position of the United Nations, a reduced relevance of the organisation.

* Is there any improvement in general of our capacity to protect the civilians in conflict and distress? What relevance do we have in disarmament, in Myanmar, Darfur, Afghanistan, Cyprus, G20....?

* I am concerned that we are in a process of decline and reduced relevance of the organisation. In short - we seem to be seen less and less as a relevant partner in the resolution of world problems.