The Government could have a problem getting legislation through Parliament to set up charter schools now Peter Dunne has said he will vote against it.
The United Future leader said today he wasn't convinced charter schools were either desirable or necessary and had told Prime Minister John Key of his decision.
Mr Dunne's party has a support agreement with the Government and his vote is crucial to its majority.
That was shown on Wednesday night when Mr Dunne voted for a Labour Party bill to "Mondayise" two public holidays - it passed on a vote of 61-60 although the Government opposed it.
If all the opposition parties vote against charter schools and Mr Dunne joins them, the Government could lose that as well.
Charter schools are a controversial ACT Party initiative, and the plan is for the first two to be set up in Auckland and Christchurch next year.
ACT and the government call them partnership schools.
They will be taxpayer-funded but run by business or community organisations.
The schools will be allowed to set their own curriculum and term dates, and don't have to hire registered teachers.
The rationale for them is that they will be able to tailor their teaching to suit the needs of under-achieving students who are failing in the state school system.
Opponents say they will be used as taxpayer-funded money making ventures.
Mr Dunne says the current system already provides a significant range of schooling options.
"I cannot see there is a need to introduce the partnership schools approach to achieve the level of flexibility the proponents of partnership schools are seeking," he said.
"United Future is extremely concerned at other aspects, such as partnership schools not needing to employ nationally certified and registered teachers, nor follow the national curriculum while at the same time being fully funded by the taxpayer."
Teacher unions are strongly opposed to charter schools and at the weekend held marches to protest against them.