Sirisena was the general secretary of the Sri Lanka Freedom Party (SLFP) but was expelled from the party by Rajapaksa ahead of polls as he broke away from the then ruling alliance in November to become the challenger in the polls.
Sirisena went on to topple Rajapaksa and end his decade-long rule in the January 8 polls.
After winning the election, Sirisena claimed the party leadership even as Rajapaksa's loyalists initially refused to back him, threatening to split the party.
In another related development, Basil Rajapaksa, the brother of Mahinda, resigned from his post of National Organiser of SLFP.
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Basil accepted full responsibility for his brother's defeat in the presidential elections, a statement said.
He led the then ruling coalition UPFA's election campaign in the run-up to the polls.
Basil left Colombo immediately after his brother lost the election. He is currently believed to be in the US on a private visit.
During Mahinda Rajapaksa's presidency between 2005 until last week, Basil was his powerful political advisor in addition to being the Minister of Economic Development.
A complaint has also been lodged against Rajapaksa and his family members over graft claims by the Marxist Janatha Vimukthi Peramuna (JVP), a key partner of the new ruling coalition.
In the run up to the polls, Rajapaksa accused Sirisena of betrayal for siding with the main opposition to be their unity candidate.
Sirisena left the Health Ministry, which he held under Rajapaksa, and also the post of SLFP's general secretary, to become the challenger in the polls.
Citing a party constitution clause that a member of the party if elected President of the country must be made the party leader, a section of the party members extended support to Sirisena as their leader.