The internal dossier, seen by the BBC, says the UN failed in its mandate to protect Sri Lanka's non-combatants because staff ''did not perceive the prevention of killing of civilians as their responsibility''. ''Events in Sri Lanka mark a grave failure by the UN to adequately respond … during the final stages of the conflict and its aftermath, to the detriment of hundreds of thousands of civilians,'' the report said.

A UN Panel of Experts report last year found up to 40,000 civilians were killed in the final months of the war, as government troops waged a brutal offensive against the separatist Tamil Tigers in the north-east of the country. The Sri Lankan government rejects that figure, saying it pursued a ''zero civilian casualty policy'', but rights groups say the number of dead may be even higher.

Under pressure from the Sri Lankan government to sanitise its reporting the UN failed to make public that ''a large majority'' of civilian deaths in the final months of fighting were caused by government shelling of hospitals and declared no-fire zones.

The report said that at UN headquarters in New York, ''engagement with member states regarding Sri Lanka was heavily influenced by what it perceived member states wanted to hear, rather than by what member states needed to know if they were to respond''. The report was also scathing of the UN's decision to pull out of the conflict zone in the north of the country.

A spokeswoman for the Australian Tamil Congress, Dr Sam Pari, said an independent and international investigation into war crimes in Sri Lanka was overdue. ''Such reports will only hold any weight if the UN acts to ensure that it redeems itself from its colossal failure in protecting the Tamil people,'' she said.