The Sri Lankan foreign minister has said that a UN panel on human rights will not be allowed into the country.
GL Peiris said that there was "no need" for the panel to come to the country and they would not be allowed in.
The UN secretary general announced earlier this week that the panel will look into alleged human rights abuses.
The UN has described the move to prevent the panel from entering the country as "most unfortunate".
"Everybody loses out if we cannot go to Sri Lanka, it will make it harder for the truth to be unearthed," former Indonesian attorney-general Marzuki Darusman - the head of the three-member panel - told the BBC.
Ban Ki-moon's spokesman said the panel would advise on how to deal with alleged perpetrators of abuses.
About 7,000 civilians died in the last five months of the war, the UN says.
There have been several allegations that both the army - and Tamil Tigers rebels who they routed last year - committed crimes at the end of the war.
Prof Peiris said that the UN panel was unnecessary.
"The position of the Sri Lanka government is abundantly clear - we will not have them in this country," he said.
Correspondents say that the government wants to fend off international concern over its conduct in the latter stages of the war - which ended in May 2009 - by launching its own internal inquiry. But its exact terms of reference are not clear.
International human rights groups are sceptical about the ability of the government to investigate claims impartially. They are demanding an independent investigation.
The UN says that its panel is designed to give advice and is not a full investigation.
Earlier this week Sri Lankan Media Minister Keheliya Rambukwella said that the government was "concerned" that Ban Ki-moon, as an outsider, had appointed the panel of human rights advisers.
Mr Darusman was part of an international team appointed to observe proceedings on a previous Sri Lankan commission on atrocities - but he resigned saying that commission did not meet basic minimum standards.