A majority of legislators from the SLPP had wanted the party to back their new ally, President Ranil Wickremesinghe, after praising him for turning the economy around after the 2022 crisis.
Wickremesinghe is not from the SLPP.
But he had its backing to replace then-president Gotabaya Rajapaksa, after he fled and resigned following months of protests over corruption and mismanagement.
Despite the downfall of Gotabaya, the SLPP enjoyed a majority in the 225-member parliament, controlled the government – and had supported Wickremesinghe.
However, the parting of ways began when the election was called last month.
That revealed the splits in the once-dominant SLPP, a nationalist party appealing to the Sinhala majority.
The SLPP said it will campaign to elect Namal as the next president, turning the presidential poll into a battle among four main candidates.
Wickremesinghe had contested two presidential elections and lost both. However, he had been prime minister six times since entering parliament in 1977.