Britain on Wednesday asked Sri Lanka to probe allegations that hundreds of men and women had been sexually assaulted by government forces during and after the island's drawn-out ethnic war.
Foreign Secretary William Hague told a public meeting in the Sri Lankan capital Colombo that Britain was concerned that no-one had been held accountable for rights abuses, including sexual assault and the use of rape to extract confessions.
He said there had been credible allegations, including those from a UN Panel of Experts, that Sri Lankan troops used sexual violence during their war against Tamil rebels and afterward.
"I will be urging the Sri Lankan government to investigate these cases as they deserve," Hague said.
The New York-based Human Rights Watch (HRW) reported earlier this year that Sri Lankan security forces continue to use rape and other forms of sexual violence to torture suspected Tamil rebels four years after the end of the island's civil war.
HRW also claimed widespread rape of detainees during the separatist conflict that ended in May 2009.
Hague said there had been "progress" in economic development, clearing land mines and re-integrating some of the former rebels back in society.
But he said no-one had been held accountable for sexual crimes during and after the war ended.
"To date, nobody has been held to account for crimes of sexual violence," he said ahead of a meeting of the Commonwealth Heads of Government in Colombo on Friday.
Canadian Prime Minister Stephen Harper is leading a boycott of the summit. The prime ministers of India and Mauritius are also staying away from the meeting over Sri Lanka's human rights record.
British premier David Cameron has also been under pressure to boycott the event, but he has promised to have "tough conversations" with President Mahinda Rajapakse about issues of accountability.
International rights groups say up to 40,000 civilians were killed by government forces in the final months of fighting in 2009, but the government has denied it killed a single civilian and has resisted calls for an international probe.