The general secretary of the People's Servants' Party, Somawanse Amarasinghe, said the resolution passed at the UN Human Rights Council last week was a betrayal of Sri Lanka.
Mr Amarasinghe, who formerly led the JVP said he would form "the widest front" against imperialist, as they had not given up their agenda of dividing the country, the state-owned Daily News reported.
"The country has been betrayed through the UNHRC resolution," he said, adding that it mandated a hybrid judicial mechanism.
He further said his party can join with the National Freedom Front and the JVP to fight separatism, if those parties showed a genuine interest against separatism.