Kenya's official opposition leader Uhuru Kenyatta has said he will not contest December's presidential poll.
Uhuru Kenyatta said he did not think he would win an election
Mr Kenyatta said the former ruling Kenya African National Union (Kanu) would instead seek an alliance with President Mwai Kibaki.
But Kanu, which ruled Kenya for 40 years, says it will field its own parliamentary and local candidates.
Mr Kenyatta, son of Kenya's first president Jomo Kenyatta, lost the 2002 election to Mr Kibaki.
This is the first time in the country's political history that the official opposition party will not contest the presidency.
"I can't just vie for the sake of vying. I can only do so when sure of winning," the Daily Nation newspaper quotes Mr Kenyatta as saying.
BBC East Africa correspondent Karen Allen says the decision by Kanu, the country's oldest party, is a significant boost to Mr Kibaki.
However, the party has been split by internal wrangles.
Kanu had been a part of the opposition Orange Democratic Movement-Kenya (ODM) but Mr Kenyatta pulled out of the coalition after a fall-out with Kanu's secretary-general William Ruto.
Mr Ruto remains a member of the Orange Democratic Movement (ODM).
Another faction, led by former party power-broker Nicholas Biwott, also broke away from Kanu because it was opposed to the party joining ODM-K, which split from ODM.
ODM and ODM-K leaders have condemned Mr Kenyatta's decision to back President Kibaki, and say he has betrayed Kenyans' trust.
ODM's vice-presidential candidate Musalia Mudavadi says the move heralds the death of Kanu.
"Kanu is on the way to winding itself up. It manifests a very serious dilemma in Kenya's political landscape and that there is a serious shortage of principle," he said.
Former President Daniel Arap Moi, who still wields a lot of influence in Kanu, last month announced that he was backing President Kibaki's re-election bid.
Opposition parties in Kenya have fragmented in recent months with the biggest presidential challenges coming from former cabinet ministers Raila Odinga and Kalonzo Musyoka.
For the 2002 poll, Mr Kibaki headed a broad alliance, including Mr Odinga, to defeat Kanu and Mr Kenyatta but this has since fractured.