The UN Human Rights Council passed a British-sponsored resolution providing for an international investigation by 23 votes to 12.
Navi Pillay, the UN high comissioner for human rights, will now be able to start an investigation into the deaths of tens of thousands of people during the final months of the civil war in 2009, when refugees from the Tamil minority were trapped and bombarded alongside rebels on a narrow stretch of beach on the north-eastern coast.
The decision came four months after the Prime Minister controversially agreed to attend the Commonwealth Summit in November, having brushed aside calls for a boycott because of Sri Lanka’s human rights record. At the time, he promised to urge an international inquiry into atrocities and disappearances if President Mahinda Rajapaksa’s own government failed to act.
Sri Lanka had vociferously opposed the measure, with the backing of China and Russia. The country said the resolution would violate its sovereignty and undermine its own efforts to heal the wounds caused by war.
But despite what British officials said was an uphill battle to secure the resolution, which required a straightforward majority vote, it was eventually passed by a comfortable margin, with 12 countries abstaining, including India and South Africa.
During an investigation in Sri Lanka last year, The Telegraph highlighted the disappearances of Anton Saniston Manuel, a fisherman who was kidnapped by suspected state agents in 2008, and Prageeth Eknaligoda, a journalist who was abducted in 2010 after criticising the president.
Mr Cameron said the passage of Thursday’s resolution was “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”.
He urged Mr Rajapaksa to cooperate with the international inquiry, adding: “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.”
Campaigners welcomed the outcome of the vote. “For five years, we and lots of other people have been banging the drum, telling people what’s going on in Sri Lanka, and it just became impossible for governments to deny it and look the other way,” said Brad Adams, the head of the Asia division of Human Rights Watch.
However, Mr Rajapaksa denounced the resolution, casting doubt over whether his government would cooperate. “We reject this,” he said. “This resolution only hurts our reconciliation efforts. It does not help.”
The resolution empowers Ms Pillay to begin a “comprehensive investigation” into “alleged serious violations” carried out by “both parties”. She can investigate abuses that took place during the period covered by a “reconciliation commission” set up by Sri Lanka’s government. That body focused on the civil war between 2002 and 2009.
She is expected to summon witnesses to help explain what happened to those missing or killed, either inside Sri Lanka or, if she is refused permission to travel there, by interviewing them abroad.
The UN has recorded 5,676 “outstanding cases” of missing people in the country – more than anywhere else in the world, apart from Iraq.