Government spending for next year is to be tabled on Friday.
Failure to pass the Bill on crucial expenditure is tantamount to the government not having majority support, which would result in its collapse.
The budget is also important as it covers the government's efforts in containing the coronavirus pandemic and restoring the economy.
The King has called for lawmakers to cease squabbling and support the budget, but some opposition MPs view the vote as a chance to test the legitimacy of Prime Minister Muhyiddin Yassin's administration.
Opposition MPs have called for a motion of no-confidence in the eight-month-old government of Tan Sri Muhyiddin, to test his actual strength in Parliament.
He has a slim majority of two seats, with 113 MPs backing him in the 222-strong Lower House.
To date, a record 25 opposition lawmakers and two government MPs have filed motions of confidence with the Speaker.
The opposition is seeking a number of reforms, including an anti-hopping law to prevent MPs from switching parties without losing their seats. Party hopping led to the end-February collapse of the Pakatan Harapan government that was voted in at the 2018 General Election.
The opposition wants all 222 lawmakers to receive equal allocations for their constituency, regardless of which party they represent. Government MPs now get RM1.5 million (S$490,000) to RM6 million annually, while opposition MPs get RM100,000 or less.
Meanwhile the government will push through with its Bill for the Independent Police Conduct Commission, which critics say does not empower the commission to take action against errant cops.