Colombo, Sri Lanka: Sri Lanka's president said on Sunday that the main reason he decided to sack his prime minister was the alleged involvement of a Cabinet minister in a plot to assassinate him.

In a televised address to the nation, President Maithripala Sirisena said a person questioned by investigators had revealed the name of a minister in an alleged plot to assassinate him and a former defence secretary.

Sri Lanka's then-incoming President Maithripala Sirisena in 2015. Credit:AP

He said the only choice for him under the circumstances was to dismiss Ranil Wickremesinghe and invite his former nemesis and ex-strongman Mahinda Rajapaksa to take over as prime minister and form a new government.

"This information (received by investigators) contains a number of details hitherto hidden to the people," Sirisena said. "The informant has made a statement regarding a Cabinet minister involved in the conspiracy to assassinate me."