Sri Lanka's President Maithripala Sirisena dashed Mahinda Rajapakse's hopes Thursday of a political comeback in next week's general election by vowing to veto any attempt by his predecessor to become prime minister.

In a letter to his former mentor, Sirisena said he would invoke executive powers to choose someone else from their United People's Freedom Alliance (UPFA) to lead the government should it triumph in Monday's polls.

"Should the party secure a majority, the prime minister should be another senior, but not you," Sirisena said in a five-page letter to Rajapakse whom he toppled as president in January.