The United Nations failed in its mandate to protect civilians in the final months of Sri Lanka's civil war, according to a draft of a damning internal UN report seen by the BBC.
"Events in Sri Lanka mark a grave failure of the UN," the draft said, adding that the world body should "be able to meet a much higher standard in fulfilling its protection and humanitarian responsibilities".
Sri Lankan forces finally crushed Tamil rebels in May 2009, following decades of brutal fighting. The conflict claimed up to 100,000 lives, according to UN estimates, and both sides are accused of war crimes.
The report blasts senior UN staff in the Sri Lankan capital, Colombo, who "did not perceive the prevention of killing of civilians as their responsibility - and agency and department heads ... were not instructing them otherwise".
It accuses the UN of failing to make public that "a large majority" of civilian deaths were caused by government shelling, a charge Colombo repeatedly denied.
The panel also questioned the UN's decision to pull out its staff from the Sri Lankan war zone in September 2008, after the government warned it could no longer guarantee their safety.
Senior UN sources said that UN Secretary General Ban Ki-moon was determined to act on the report's recommendations in order to "learn lessons" from Sri Lanka and respond more effectively to new international crises.