Kofi Annan has submitted his resignation as special envoy to Syria, saying he plans to step down on August 31.
Secretary-General Ban Ki-moon said he accepted the resignation with "deep regret".
Mr Annan called a last-minute press conference in Geneva on Thursday afternoon and delivered blistering criticism of world powers' failure to unite over the country's escalating violence.
He said he accepted the role when it seemed the international community led by the UN Security Council could help end the violence, enforce a cease-fire and bring about a political transition.
But the former UN secretary-general told reporters he cannot go on when the 15-nation council provides no backing for his role, particularly because of the standoff between its five veto-wielding members: Russia and China on one side, the United States, Britain and France on the other.
He was appointed special envoy in February of last year.
"When the Syrian people desperately need action, there continues to be finger pointing and name calling in the Security Council," he told reporters. "It is impossible for me or anyone to compel the Syrian government and also the opposition to take the steps to bring about the political process. As an envoy, I can't want peace more than the protagonists, more than Security council or the international community, for that matter," he added.
Annan said the failed six-point plan commonly referred to as the Annan plan is, in fact, the Security Council's plan.
He did not rule out the idea of a successor being appointed by the current UN secretary-general, Ban Ki-moon, since "the world is full of crazy people like me, so don't be surprised if someone else decides to take it on."
Ban said in a statement he is discussing possible successors with the Arab League.