"The international community must realise that this is a democracy. They must understand our position. We are seeking a mandate from the people," Rajapaksa was quoted by AFP.
According to PTI, the former president obtained the membership of the SLPP, launched by his supporters, Sunday morning. SLFP was founded in 1951, his father Don Alwin Rajapaksa was a founding member of the party.
The SLPP was formed last year by Rajapaksa's supporters to create a platform for his re-entry into politics. The party in February's local council election won two-thirds of the total 340 seats.
The ongoing political turmoil in Sri Lanka began around two weeks ago after President Maithripala Sirisena sacked prime minister Ranil Wickremesinghe, replacing him with Rajapaksa. The power swap move was described as illegal by rivals.
Sirisena dismissed parliament on Friday and called new polls for January 5, throwing the island into fresh chaos as the United States urged Sri Lanka to respect democratic processes, news agency AFP reported.
Wickremesinghe's party had wanted parliament reconvened to prove he commanded a majority, but Sirisena refused and sacked the legislature instead, escalating a standoff between the rival factions.