Canadian Prime Minister Stephen Harper said weeks ago he was skipping the meeting because of concern about a deterioration in human rights and his Indian counterpart, Manmohan Singh, has also pulled out.
British Prime Minister David Cameron says he will ask "serious questions" and demand an investigation into allegations of war crimes.
Separatist rebels from the minority Tamil community battled government forces for 26 years, beginning in 1983, until they were finally crushed in a big army offensive in 2009.
A U.N. panel concluded that thousands of civilians, most of them Tamils, died in the final offensive. It concluded that atrocities were committed by both sides, but that most of the
victims were killed by army shelling.
The government says Sri Lanka is on the path to reconciliation, helped by fast economic growth.
But complaints of rights abuses, such as those recently aired in a forum organised by Human Rights Watch, have raised concern about the peace.
Two ethnic Tamils, speaking from Britain where they are seeking asylum, told reporters last month they had been detained by security forces, repeatedly raped and beaten in what rights groups say is a pattern of intimidation.
One of them said he was stopped on the street and pushed into a van last year. During five days in detention, a group of men beat him with a plastic pipe and repeatedly inserted a metal rod into his rectum, he said.
"I had no choice, I couldn't stand the torture so I admitted to the allegation I was in the LTTE," the man said, referring to the now disbanded Liberation Tigers of Tamil Elam (LTTE) rebels.
A 30-year-old woman said in the conference call she was pushed into a van last August and raped in detention for 19 days by different men.
"While in detention I could hear other women screaming in other rooms," she said. Both said they were released after friends and relatives paid a ransom.
Human Rights Watch investigator Charu Lata Hogg said evidence of rape had been detected by doctors, but rarely included in asylum applications because of stigma. She said she had documented 75 cases.


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