A survey of teachers has found widespread concern about how those with a traditional understanding of marriage will be treated following the legalisation of gay marriage.
The ComRes poll found that more than half (56%) were "concerned" or "very concerned" that expressing support for traditional marriage could damage their professional career.
One in ten said they would probably refuse to teach about gay marriage, while just under a fifth (17%) said they would probably teach the importance of gay marriage but "wouldn't be happy about it".
However, just under half (49%) said parents should not have the legal right to withdraw their children from lessons covering same-sex marriage.
John Bowers QC has said the legal requirement for schools to promote marriage would place a "duty" on teachers to promote the new legal definition unless the Education Act 1996 were amended.
"This provides no exception for conscientious beliefs," he said.
"The stark position in my view is that a Christian teacher (or indeed any teacher with a conscientious objection) may have to teach about (and positively portray) a notion of marriage (and its importance for family life) which they may find deeply offensive.
"If the Marriage Bill becomes law, schools could lawfully discipline a teacher who refused to teach materials endorsing same sex marriage."
Colin Hart, Campaign Director for Coalition for Marriage said: "Tens of thousands of teachers face the real prospect of being disciplined, or sacked, over the Government's proposals to redefine marriage, creating a poisonous atmosphere in every staffroom in every school.
"The legislation contains no safeguards for those who work in the public sector."