Sri Lanka's foreign minister has launched a bitter attack on the prime minister, saying he has betrayed the principles of the ruling party.
Bandaranaike (L) was set to be PM under a president Rajapakse
Foreign Minister Anura Bandaranaike said PM Mahinda Rajapakse had joined with "extremist forces" to try to win this year's presidential elections.
Last week Mr Rajapakse signed a deal with the Sinhala nationalist JVP.
Mr Bandaranaike, brother of President Chandrika Kumaratunga, said he "didn't care" about the poll outcome now.
The ruling Sri Lanka Freedom Party had earlier agreed Mr Bandaranaike would be prime minister under Mr Rajapakse if it won the election.
Mr Bandaranaike was speaking to the country's Daily Mirror newspaper.
He said: "I really don't care about the outcome of the election now, since the party's long upheld principles have been betrayed.
"Our party's candidate has joined hands with extremist forces and understandably I and my sister strongly feel major damage has been done to the party by his actions."
Mr Rajapakse has not commented on Mr Bandaranaike's outburst.
Analysts say the spat could harm the premier's campaign and push moderates to vote for former premier Ranil Wickramasinghe of the United National Party who is battling Mr Rajapakse for the presidency.
Prof Jayadeva Uyangoda, of Colombo University's political science section, told Reuters: "This is very damaging to Rajapakse's campaign. With all the internal politics in his own party, it is going to be very difficult for the prime minister."
On Tuesday, Mr Rajapakse followed up his pre-poll pact with the JVP by sealing a deal with another Sinhala nationalist group - the all-monk Buddhist party, the Jathika Hela Urumaya (JHU).
Both the JVP and JHU have been longstanding critics of the handling of peace moves by President Kumaratunga with the country's Tamil Tiger rebels.
Mr Bandaranaike told the Daily Mirror he thought the JVP had let the president down over the peace process.
Mr Rajapakse's deal with the JVP included a commitment to retain a unitary state in Sri Lanka, redraft a ceasefire with Tamil Tigers and to stop privatisation.
In return the JVP, who pulled out of the government this year complaining about concessions to the Tigers, would support Mr Rajapakse's election bid.
President Kumaratunga has defended her policies, vowing she would not "throw away 11 years of work on a mere election promise".