The United People's Freedom Alliance (UPFA) decided to vote against the budget during the second reading vote today.
A spokesman of the party said the UPFA group supporting President Maithripala Sirisena arrived at the decision a short while ago.
The second reading vote of the budget will take place in Parliament later today.
However, UPFA MP Shantha Bandara, a supporter of President Sirisena, said the UPFA group had unanimously decided to vote for the President's Expenditure Head.
A group of UNP backbenchers have already threatened to vote down the President's Expenditure Head in Parliament.
The move, however, has not elicited a positive response from UNP senior holding cabinet, state and deputy ministerial positions.