David Cameron has urged Sri Lanka to co-operate with a United Nations-led inquiry into alleged civil war atrocities after a UK-sponsored call for a independent probe was approved by a human rights body.
The Prime Minister, who clashed with president Mahinda Rajapaksa on the issue during a visit to the island last year, said it was in Sri Lanka's interests to "seize this fresh opportunity to work with the international community to heal the issues of the past".
Despite opposition from China and other members of the 47-nation body meeting in Geneva, a resolution empowering UN high commissioner for human rights Navi Pillay to conduct an investigation was adopted by 23 votes to 12, with 12 abstentions.
There should be an update in September and a "comprehensive report" within a year, it says, at an anticipated cost of 1.46 million dollars (£880,000).
As many as 40,000 civilians are estimated to have died in the final months of the Sri Lankan government's battle with Tamil Tiger separatists, with both sides accused of widespread human rights abuses, including executions and rapes of prisoners by state troops.
The Rajapaksa regime is also accused of continued post-war human rights violations amid a surge in religious attacks, concern over the independence of the judiciary and evidence of intimidation and harassment of human rights defenders.
Mr Cameron said: "I am pleased that the UN Human Rights Council has today agreed to press ahead with its own independent investigation into alleged war crimes in Sri Lanka.
"This is a victory for the people of Sri Lanka who need to know the truth about what happened during those terrible years of the civil war so that they can move forward.
"Today's outcome has been triggered by the failure of the Sri Lankan government to stand by its promises to credibly and independently investigate alleged violations on both sides during the war.
"I hope that president Rajapaksa will now seize this fresh opportunity to work with the international community to heal the issues of the past and to reconcile communities across Sri Lanka. This is the best way to achieve an inclusive and prosperous future and to ensure that Sri Lanka achieves its amazing potential
"I am proud of the crucial role that Britain has played to secure this outcome. I said in November that we would achieve more by standing up for our values rather than sitting on the sidelines. And that is exactly what we have done."
On a visit to the island in November for a Commonwealth summit, the PM warned Mr Rajapaksa during a stormy meeting that he had until this month to set up a credible domestic inquiry.
A probe into 16,000 people still listed as missing has been dismissed by critics as a sham and there has been little or no progress on promises to set up a truth and reconciliation commission.
Ms Pillay has concluded Colombo does not have the will to investigate properly.
Mr Cameron confronted the president after meeting displaced Tamils who have spent more than 20 years in a temporary camp, during the first visit in 60 years by a world leader to the battle-scarred north.
An angry Mr Rajapaksa, who denies claims of war crimes and continuing abuses against the minority Tamils, accused Mr Cameron of seeking votes from the UK's large Tamil community.
Countries opposing the resolution were: Algeria; China; Congo; Cuba; Kenya; Maldives; Pakistan; Russia; Saudi Arabia; United Arab Emirates; Venezuela; and Vietnam.
The British Tamils Forum gave a qualified welcome to the vote.
" British Tamils Forum and many other organisations have expressed concern regarding the ability of an OHCHR-led inquiry to deliver justice to the victims and survivors," it said.
"We also regret the unduly narrow scope of the inquiry and the resolution's failure to note the ethnocratic dimension to the conflict.
"Nevertheless, we believe this resolution represents a step forward in the Tamil people's quest for justice, rights and freedom.
"British Tamils must work with our allies around the world to strengthen this inquiry, deliver witnesses and evidence, and pursue our campaign for justice until Sri Lanka's genocide of the Tamil people is halted."
Edward Mortimer, chair of the Sri Lanka Campaign for Peace and Justice, said: "It has been obvious for at least three years that only an independent international investigation can establish the truth about what happened in Sri Lanka in the last months of the civil war.
"There is credible evidence that both sides committed crimes against humanity, and the lack of accountability for this has engendered a culture of impunity which has fled to further outrages. We believe that these too amount to crimes against humanity, and that they persist to this day.
"The High Commissioner for Human Rights now has a clear mandate to conduct a robust and thorough investigation into what took place, and is taking place, in Sri Lanka. It is important that she, and her successor, make full use of this mandate.
"The international community must not now look away from Sri Lanka.
"In the last two weeks we have seen a severe crackdown on all forms of dissent. Campaigner for the Disappeared Balendran Jeyakumari, and at least 10 others, remain in detention without access to their lawyers, and no evidence has been produced against them.
"Meanwhile Ruki Fernando and Father Praveen Mahesan, the activists who were released last week, are still subjected to a gagging order and other forms of judicial harassment.
"It is terrifying to think how much further the Sri Lankan authorities will go if the international community's attention moves on. They need to be told firmly that there will be real consequences if the crackdown continues."