Sri Lankan President Gotabhaya Rajapaksa's elder brother and former strongman Mahinda Rajapaksa will be sworn in as the new prime minister on Thursday.

Gotabhaya on Wednesday named Rajapaksa as the new prime minister after incumbent Ranil Wickremesinghe announced his resignation from the post following the election debacle.

Rajapaksa, the main opposition leader, will be appointed as prime minister after incumbent Wickremesinghe steps down formally on Thursday, his spokesman Rohan Weliwita was quoted as saying by Times Online.

The former president, who will function as Prime Minister of the caretaker cabinet until the general election, is expected to be sworn in at 1 pm before assuming office at around 3 pm, it added.

Rajapaksa was appointed the prime minister on October 26, 2018 by the then President Maithripala Sirisena, who sacked Wickremesinghe in a controversial move that plunged the country into an unprecedented constitutional crisis.

He resigned in December as two crucial Supreme Court decisions made the former strongman's efforts to cling to premiership untenable.

The apex court later unanimously declared that the dissolution of Parliament by Sirisena was "illegal".

Rajapaksa won power in 2005 and went on to become South Asia's longest-serving leader. He became the country's youngest ever parliamentarian in 1970 at the age of 24.

The two brothers -- Rajapaksa and Gotabaya -- led a decisive campaign that helped end the island nation's three decade long civil war against the Liberation Tigers of Tamil Eelam (LTTE).

On Wednesday, Wickremesinghe announced his resignation, days after the ruling party's candidate lost the presidential election to Gotabaya.

Gotabaya defeated Wickremesinghe's deputy Sajith Premadasa.