There is "enormous concern" among teachers and parents at the influence of some religious "fundamentalists" on education, a union leader says.
The issue goes beyond faith schools, NUT leader says
Education unions' annual conferences will debate next week calls for an end to state funding for faith schools.
National Union of Teachers general secretary Steve Sinnott said there was a view that faith groups could damage social cohesion.
The government says the opposite is the case, and faith schools are popular.
The potential for religious groups to sponsor more state schools is one of the concerns raised by critics of the government's latest education reform plans.
'Millions to spend'
"There is enormous concern, I think, around the country," Mr Sinnott said at a pre-conference briefing in London.
It focused on the influence of groups with "views on religion "outside the mainstream - sometimes described as fundamentalist".
"It's more than just faith schools," he said.
Some school sponsors - "because they have millions of pounds they wish to spend on education" - could "present to youngsters in the classroom their prejudice and their narrow views on religion".
Mr Sinnott suggested the union was likely to begin an investigation into the issue.
A motion tabled for the conference in Torquay, starting next week, calls for "a long-term phased programme of ending state funding to faith schools".
"All such schools should be given government encouragement and support for moving back to full integration within the state sector."
And delegates at the Association of Teachers and Lecturers conference will be debating a similar resolution.
It says the government's policy of having more faith schools "will hinder integration, foster religious divisions and provide fertile ground for religious and ethnic conflicts."
The NUT has reiterated its opposition to the government's programme of having 200 city academies by 2010.
Academies are independent state schools backed by outside sponsors, who put in £2m in return for a majority stake in their running.
It has been claimed that schools in the North East backed by Christian millionaire Sir Peter Vardy have been teaching Creationism alongside evolutionary scientific theory.
Some NUT delegates want the teaching of "creationism or intelligent design as a valid alternative to evolution" to be outlawed.
A spokesman for the Department for Education said: "We have a long tradition of faith schools in this country."
He added: "They are popular with parents and make an important contribution to community cohesion by promoting inclusion and developing partnerships with schools of other faiths, and with non-faith schools.
"Leaders from the Church of England, Hindu, Sikh, Catholic, Muslim, Jewish and Buddhist faiths only recently underlined their commitment to ensure that faith schools teach pupils about other religions as well as their own."