Sri Lankan Prime Minister Ranil Wickremesinghe will resign on Thursday, his office said on Wednesday, days after the ruling party's candidate lost the presidential election to Gotabhaya Rajapaksa.
"Wickremesinghe will resign from his position tomorrow," his office said.
He was under pressure to quit and handover the government to the opposite camp after Rajapaksa won the presidential election on Saturday.
Rajapaksa defeated Wickremesinghe's deputy Sajith Premadasa.
Sources said President Rajapaksa will appoint his elder brother and former strongman Mahinda Rajapaksa, the current main opposition leader, as the next prime minister.
Mahinda was appointed the Prime Minister on October 26, 2018 by the then President Maithripala Sirisena, 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 embattled former strongman's efforts to cling to premiership untenable.
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The apex court later unanimously declared that the dissolution of Parliament by Sirisena was "illegal".
Mahinda 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 United National Party (UNP) leader faced a revolt since Premadasa's defeat, a regular occurrence during the last 25 years whenever the UNP lost a major election. Yet Wickremesinghe held on.
Harin Fernando, a minister from the party's younger brigade, told reporters that they want Wickremesinghe to quit the party leadership and name Premadasa his successor for both the party leadership and the post of the main opposition leader.
"We will form our own party if he did not make the necessary changes this time," Fernando told reporters.
There will be a caretaker cabinet of 15 members to run the government until Rajapaksa will be constitutionally able to dissolve parliament after February 2020.