Sri Lanka's prime minister Mahinda Rajapaksa will resign from his post, his son said on Friday, in a move that will force President Maithripala Sirisena to appoint a replacement.

"To ensure stability of the nation, former president ... Rajapaksa has decided to resign from the Premiership tomorrow after an address to the nation," Namal Rajapaksa, an MP in his father's party, said on his Twitter account.