On Thursday he said the former strongman leader would defect from the Sri Lanka Freedom Party (SLFP) that he once led to head the new party.

Sirisena formed a unity government with the right-wing United National Party after taking power and is the current head of the SLFP, but more than half its MPs remain loyal to Rajapakse.

“Our leader is Mahinda Rajapakse,” Basil Rajapakse told reporters in Colombo. “It is the Mahinda Rajapakse vision that drives us.”

Rajapakse tried to become prime minister at parliamentary elections held seven months after his shock defeat, but failed and now pulls strings from behind the scenes as a backbencher.

A formal break-up of the SLFP could deprive Sirisena of the two thirds majority required for constitutional reforms he has promised to bring about to ensure reconciliation after decades of ethnic war.

Sirisena’s administration is currently working on a new constitution to share political power with the island’s Tamil minority and end decades of ethnic unrest.

Rajapakse, who turns 71 on Friday, has declined to say whether he will formally quit the SLFP, but the new party – Sri Lanka People’s Front (SLPF) – is being launched ahead of local government elections due early next year.

There was no immediate comment from Sirisena, who has previously warned Rajapakse against breaking up the SLFP. Soon after coming to power in January last year, Sirisena ordered criminal investigations into corruption under the Rajapakse regime.

The former leader’s closest family members face allegations of bribery, extortion and even murder.

Rajapakse’s two sons – legislator Namal and ex-navy officer Yoshitha – and two of his siblings are currently on bail after being charged with corruption, including money laundering. Sri Lanka has sought help from the US Federal Bureau of Investigations and other international organisations to track down billions of dollars said to have been siphoned out of the island during Rajapakse’s decade in power.