In a letter to UNHCR chief Zeid Ra'ad Al Hussein, slain LTTE supremo V Prabhakaran's relative M K Shivajilingam said UN's failure to protect Tamils in 2009 was dubbed a "systematic failure" in its own report in November 2012.
Hussein is scheduled to release a report investigating the warcrimes, at the UNHRC in Geneva next month.
"As you prepare the report on Sri Lanka, I would like to request you to consider the pattern of historic failures of Sri Lanka against Tamil people," Shivajilingam wrote.
He is running a team of independents from the north western district of Kurunegala. Rajapaksa, too, has move to Kurunegala from his native south to cash in on the Sinhala- Buddhist votes in the district.
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Shivajilingam said the Tamil people do not believe that anything short of an international judicial process will be seen as justice.
The UNHRC adopted a resolution in 2014 advocating an international investigation into the possible warcrimes by Sri Lankan forces and the Tamil Tiger rebels in the final stages of the 26-year civil war that ended in 2009.
Tamils remain skeptical about the domestic investigation.
The UN has delayed its report until September from March, which, it said, was to allow more time to Sirisena government.
According to a 2011 report by the UN, about 40,000 Tamil civilians were killed in the final weeks of the civil war.
The previous Rajapaksa government did not collaborate with the international investigation, calling it an attack on Sri Lanka's sovereignty.