Amnesty International has criticised the "politicisation of international justice" in its annual report, which documents torture in 111 countries.
The human rights group accuses powerful governments of subordinating justice to political self-interest and of shielding allies from scrutiny.
It expresses particular concern over possible war crimes committed during fighting in Sri Lanka last year.
The report also criticises the UN for its failure to intervene.
Thousands of people were killed during the war, and a UN spokesman described the situation in northern Sri Lanka at the time as a "bloodbath".
But Amnesty says that "power plays" at the UN Human Rights Council led to member states approving a resolution drafted by the Sri Lankan government, complimenting itself on its success against the Tamil Tigers.
"By the end of the year, despite further evidence of war crimes and other abuses, no-one had been brought to justice," Amnesty's Secretary General Claudio Cordone says. "One would be hard pressed to imagine a more complete failure to hold to account those who abuse human rights."
In its report, Amnesty also cites the United States and European Union for using their position with the UN Security Council "to continue to shield Israel from strong measures of accountability for its actions in Gaza".
But it says that there have been positive developments over the past year as well.
In Latin America, it notes the conviction of former President Alberto Fujimori of Peru for crimes against humanity, and Argentina's last military President Reynaldo Bignone for kidnapping and torture.
And it hails the arrest warrant issued by the International Criminal Court (ICC) for the Sudanese President, Omar al-Bashir, on war crimes charges as a "landmark event".
However, it is disappointed by the African Union's failure to co-operate with the ICC, despite a "nightmare of violence" in Darfur affecting hundreds of thousands of people. This, it says, is a "stark example of government failure to put justice before politics".
And it calls on all G20 countries - including the US, China and Russia - to sign up to sign up fully to the ICC.
Pressure on Britain
Amnesty is also highly critical of the previous British government for "stonewalling" on repeated calls for an independent investigation into allegations that UK intelligence officials were complicit in torture, "rendition" and secret detention.
The organisation is also concerned about the UK relying on "diplomatic assurances" when carrying out deportations to countries such as Algeria and Jordan.
Last week, new Foreign Secretary William Hague promised that an inquiry would be held into allegations of complicity in torture. But few details were released, and the Foreign Office says the issue is still being discussed by ministers in the National Security Council.
Amnesty has welcomed Mr Hague's pledge.
"We look forward to an inquiry that is truly independent and looks not only at potential criminal responsibility but also at Britain's co-operation agreements with the United States and other countries," says Mr Cordone. "It should leave no stone unturned."