Sri Lankan Prime Minister Ranil Wickremesinghe will resign from office on Thursday after his party's candidate lost a presidential election over the weekend, his spokesman said.

An interim cabinet, appointed by new president Gotabaya Rajapaksa, is expected to govern the island until the next election which could be around April, four sources from both Rajapaksa and Wickremesinghe's teams told Reuters.

"Prime Minister Ranil Wickremesinghe announced that he has decided to resign," spokesman Sudharshana Gunawardena said. "The resignation letter will be sent (to the President) tomorrow."