COLOMBO - Sri Lanka's central bank chief Indrajith Coomaraswamy said on Monday he was resigning and would step down on Dec. 20, citing personal reasons and the age limit for staff of regulated entities.

"I informed relevant people well before the election," Coomaraswamy, 69, told Reuters from Singapore in a Whatsapp message, confirming local media reports.

Former wartime defence chief Gotabaya Rajapaksa won a Nov. 16 presidential election and has since appointed his elder brother, former president Mahinda Rajapaksa, as prime minister of a new government.

Coomaraswamy, a Sri Lankan Tamil, joined the central bank in 1974 and become governor in 2016, helping steer the economy as it was hit by flooding and droughts and twin debt and balance of payments crises.

This year, the bank slashed interest rates to support growth after tourism and investment plunged following the Easter bomb attacks by Islamist militants that killed more than 250 people.

Analysts said Coomaraswamy had brought certainty to the conduct of monetary policy and other functions of the central bank following the relatively brief tenure of the previous governor, who quit after corruption allegations.

It was not immediately clear who will succeed Coomaraswamy but central bank sources said senior deputy governor Nandalal Weerasinghe is an option if Rajapaksa opts for a career central banker.

(Reporting by Ranga Sirilal and Shihar Aneez; Editing by Catherine Evans) ((; +94-11-232-5540; Reuters Messaging: ;