The report was unusually scathing for an official U.N. critique.
The report accused U.N. staff in Colombo of not perceiving that preventing civilian deaths was their responsibility and accused their bosses at U.N. headquarters of not telling them otherwise. A separate U.N. report released last year said up to 40,000 ethnic minority Tamil civilians may have been killed in the war's final months.
''This report is a benchmark moment for the U.N. in the same way that Rwanda was," said Gordon Weiss, a former U.N. spokesman in Sri Lanka.
The report accused U.N. officials and member states of being reluctant to interfere and leaving the conflict in a ''vacuum of inaction."
"The report concludes that the United Nations system failed to meet its responsibilities—highlighting, in particular, the roles played by the Secretariat, the agencies and programs of the U.N. Country Team and the members of the Security Council and Human Rights Council," Secretary-General Ban Ki-moon said in a statement.
The report was compiled by a committee headed by former U.N. official Charles Petrie. It investigated U.N. actions as the quarter-century war between the government, dominated by the ethnic Sinhalese majority, and minority Tamil rebels ended in 2009 in a wave of violence.
The BBC first reported on a draft of the report Tuesday.
The draft, obtained by The Associated Press shortly before the final report was released, began with an "executive summary" that detailed the U.N. failure on the ground, saying that the political conditions after the Sept. 11, 2001, terror attacks in the U.S. made countries less likely to stop a government fighting against a group—the Tamil Tiger rebels—that many had branded a terrorist organization.
The executive summary was deleted from the official published version issued on Wednesday.
The draft report painted a picture of a U.N. operation reluctant to criticize the government or accuse it of killing civilians with artillery bombardments, out of concern the government would respond by limiting U.N. humanitarian access—even through U.N. aid workers were barred from the northern war zone in late 2008.
Top U.N. officials in the country repeatedly worked to soften statements to remove casualty figures and accusations of possible war crimes against the government, the official report said. When death tolls its staff was compiling were released, top officials dismissed them as unverified despite the rigorous methodology being used, the report said.
When U.N. satellite images confirmed heavy artillery shelling in the war zone and showed far more civilians there than the government claimed, the top U.N. official in Sri Lanka downplayed the evidence in a letter to the government, the final version of the report said. At the same time, member states did not hold a single formal meeting on the conflict in its final months in the Security Council, Human Rights Council or the General Assembly.
''The UN set itself up for failure, in Sri Lanka," the draft report said, in another quote deleted from the final, official report.
The report also accused the government of working to intimidate U.N. staff, of withholding visas of those critical of the government and of planting false allegations against them in the media.
The U.N. representative for Human Rights Watch, Philippe Bolopion, said "The U.N.'s dereliction of duty in Sri Lanka is a stark reminder of what happens when human rights concerns are marginalized or labeled as too political."
Nessman reported from New Delhi.