Bandaranaike is the brother of former president Chandrika Kumaratunga. He was sacked in February by President Mahinda Rajapakse along with two other ruling party lawmakers.
But Bandaranaike returned soon after and took back his ministerial portfolio after patching up differences with Rajapakse.
Bandaranaike's latest crossover has increased uncertainty over the fate of the government.
The government could fall if it fails to get its 2008 budget passed by parliament at the final vote Friday evening.
A defeat of the budget dissolves the cabinet of ministers, who could be re-appointed by the president and the budget re-presented.
A previous vote was won comfortably by the government with 118 votes against 102.
Earlier in the week four members of a minority party