Mr Rajapaksa's son on Friday announced that the former strongman will resign on Saturday, after two crucial Supreme Court decisions made the embattled leader's efforts to cling to premiership untenable.
Namal Rajapakse said they will join a coalition with Sirisena who triggered the political crisis on October 26 by initially sacking Prime Minister Ranil Wickremesinghe and replacing him with former foe Rajapakse.
Sri Lanka's Supreme court on Friday refused to stay a court order restraining Rajapaksa, 73, from holding the office of Prime Minister until it fully heard the case next month. That issue is now being canvassed before courts.
The President had a meeting with Speaker Karu Jayasuriya and Ranil Wickremesinghe yesterday (13th) and has reportedly agreed at the meeting to reinstate Wickremesinghe as the prime minister. This comes after a day after the Sri Lankan Supreme Court unanimously ruled that the dissolution of Parliament by Srisena was illegal.
Two of the country's highest courts suspended Rajapakse till he could prove his legitimacy after he failed two no-confidence motions in the 225-member legislature in mid-November.
Political sources said Sirisena was likely to invite Wickremesinghe to form a new government over the weekend and end the power vacuum.
The petitioners, the Members of Parliament representing the United National Party, Tamil National Alliance, Janatha Vimukthi Peramuna, Sri Lanka Muslim Congress and several other parties filed this petition naming 49 persons including Prime Minister Mahinda Rajapaksa, members of Cabinet Ministers and Deputy Ministers as respondents.
Parliamentarian Lakshman Yapa Abeywardena said Rajapaksa, who ruled Sri Lanka for almost a decade, made a decision to step down in the best interest of the country.
Wickremesinghe's United National Party (UNP) said that it was decided during the discussion that he will take oath as the Prime Minister on Sunday.
However the relationship between president and prime minister turned sour and Mr Sirisena in October turned on Mr Wickremesinghe, sacking him in favour of Mr Rajapaksa, his old ally-turned-rival-turned-ally.
President Sirisena, meanwhile insisted that he will never appoint Wickremesinghe again as Prime Minister.