Opposition Assembly members are threatening to thwart Labour's spending plans because they want more cash in 12 areas, including schools and universities.
The minority Assembly government tabled its draft budget for 2007-8 this week, but ministers failed to satisfy opposition leaders. In particular, they want to see 1 per cent efficiency gains imposed on schools returned to heads.
That proposal was welcomed by Anna Brychan, director of the National Association of Head Teachers Cymru, which has warned that budget cuts by councils could result in teacher redundancies and larger class sizes.
She said: "We welcome any move to give more funding direct to schools. But what is euphemistically termed an 'efficiency saving' is in fact a spending cut.
"Giving schools the 1 per cent would give heads greater flexibility but it would do so in the context of a shrinking budget overall."
At a joint press conference, the leaders of Plaid Cymru, the Tories and the Lib Dems said they would vote down the budget and table an amendment when the Assembly votes next Tuesday.
But finance minister Sue Essex said she had stretched the government's coffers to the limit while drafting the pound;14 billion budget, up pound;74 million on the current year.
The 2007-8 budget completes a three-year cycle that has provided pound;670m for school buildings, free school breakfasts at a quarter of primaries, free summer holiday swimming for children, and no variable fees for Welsh university students, she added.