Colombo repeated President Mahinda Rajapakse's protest to Ban in March that the panel was "unwarranted and uncalled for," a senior government official said, declining to be named.
"The government is deeply unhappy with the appointment of this panel and made it very clear to the secretary-general himself and other UN representatives that this is unwarranted and uncalled for," the official said.
Ban is due to name a three-member panel Monday to advise him on the Sri Lankan military campaign that crushed the separatist Tamil Tiger rebels after 37 years of fighting.
The government has been dogged by war crime allegations in the wake of the campaign.
The exact mandate of the panel is not yet clear, but diplomats said the team could be a precursor to a full-blown war crimes investigation.
The UN has said that at least 7,000 ethnic Tamil civilians perished in the first four months of last year, just before the government claimed final victory over the Tigers.
The military has al