COLOMBO - Former Sri Lankan Prime Minister Ranil Wickremesinghe will return to the post on Sunday, a lawmaker from his party and a source at the president's office said, likely ending a political crisis that began in late October when he was ousted in a surprise decision.

The South Asian island country had plunged into instability after President Maithripala Sirisena replaced Wickremesinghe with Mahinda Rajapaksa, who was then twice sacked by parliament. Rajapaksa resigned on Saturday as a government shutdown loomed.

"He (Wickremesinghe) will take oath at an auspicious time today morning," Rajitha Senaratne, a cabinet spokesman under Wickremesinghe's former government, told Reuters.

An official at the president's office confirmed Wickremesinghe's oath taking.

(Reporting by Shihar Aneez; Editing by Krishna N. Das) ((Krishna.Das@tr.com; +91-98711-18314, +91 11 4954 8026; Reuters Messaging: Krishna.Das.thomsonreuters.com@reuters.net, Twitter: https://twitter.com/krishnadas56))