"Mahinda Rajapaksa who lost on January 8, will be defeated again. I will not make him the Prime Minister even if my party wins the election," Sirisena said in a televised address.
Sirisena, who came under fire for allowing Rajapaksa to contest election, was making his first public appearance since his controversial decision which angered most of his supporters. It even led to the formation of a new anti-Rajapaksa coalition.
"I was all along opposing Mahinda Rajapaksa's nomination. I flatly rejected the proposal to make him the prime minister," Sirisena said.
"I finally told them to do whatever they wished," he said, adding that he did not want to cause a division in the party by being too adamant.
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Sirisena said he was forced to dissolve the parliament and order the August 17 poll to defeat a ploy by the pro- Rajapaksa camp to make him the Prime Minister.
The section of Sirisena's party who backed Rajapaksa had moved a motion of no confidence against his minority Prime Minister Ranil Wickremesinghe in order to make Rajapaksa the Prime Minister.
Sirisena said he was protecting the unity of the party as the political history of the island points to oppressive regimes whenever one of the two main parties suffered a split and weakened as a result.