Jayantha Dhanapala, a diplomat and former candidate for the post of UN secretary general who was appointed as peace advisor in December 2005, said he was quitting for "personal reasons," in a statement released by his office.
The move followed the government's decision to withdraw from a 2002 truce, despite mounting international concern that the island's military could slip back into full-scale war with Tiger guerrillas.
Close confidants of Dhanapala said he was embarrassed by the turn of events, with the government pressing for a military victory after scoring several battlefield successes against the rebels last year.
There was no immediate reaction from the government, which last week announced it would quit the moribund truce from January 16. Dhanapala had also served as head of the government's peace secretariat, which coordinated the Norwegian-led peace efforts with Tamil separatists to end the decades-long conflict, which has killed tens of thousands.