Rules which allow teachers to spend paid time on union work rather than being in the classroom are to be reviewed.
In some cases, union representatives are spending all of their time on union activities instead of teaching lessons, the Department for Education (DfE) said.
Schools minister David Laws said there are concerns that taxpayers are funding union officials who spend too little time teaching.
One union said there should be good relationships between schools and trade unions, but that at a time of squeeze on budgets, it was appropriate to look at the costs of all activities, including funding time for trade union representatives.
But another said trade unions play a “vital role” and that it should not be for the Government to specify how unions organise themselves.
Under the current system, trade union officials can take reasonable paid time off, of “facility time”, to take part in their union work, such as representing their members in employment disputes and negotiating with schools.
The DfE insisted it is not proposing to change the rules, but said that the interpretation of what is reasonable “varies widely”.
It said that there is evidence that a “large number” of union officials are paid as teachers, but do not spend any of their time teaching, which the department said creates extra work for other school staff.
“Some are nominally employed as ‘home tutors’ by their local authority, while others receive large salaries on the teachers’ scale (in some cases up to £70,000 a year) but have no teaching timetable or leadership responsibilities,” the DfE said.
“This represents a significant cost to the taxpayer of many millions of pounds.”
Employers, headteachers, teachers and the public are being asked to submit evidence on facility time to the Government by October 25.
Mr Laws said: “Effective representation of teachers can play an important role in schools, but for too long teaching unions have received taxpayer funding with little transparency to the public about how this is spent. We believe substantial efficiencies can be found.
“I believe that it is reasonable to expect all trade union representatives in schools to spend the majority of their working hours carrying out their school-based jobs. Trade union representatives should be grounded in classroom practice, and should not be spending all their taxpayer-funded time out of the classroom.”
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said: “ASCL is firmly of the view that there should be excellent working relationships in schools between school leaders and all trade unions and their representatives. We acknowledge that facilities time funding frees up union representatives to promote these good working relationships.
“Nevertheless, at a time when resources for schools are very tight, it is appropriate to examine the costs of all activities including the funding of time for trade union officials.”
But Russell Hobby, general secretary of the National Association of Head Teachers, said: “Trade unions can play a vital role in any workplace. There is strong evidence that they increase productivity and save the taxpayer money by preventing conflicts and tribunals as well as by promoting training and development.
“This is certainly true in education, where many head teachers will tell you that they maintain constructive relationships with the unions and find them a helpful source of feedback on policies and the views of teachers.
“It is not for the Government to specify how unions should organise themselves or choose their representatives.”