If gay marriage is legalised teachers and others could be forced out of their jobs if they fail to endorse such unions, a top lawyer says.
Parents would have no right to insist that their children are withdrawn from school lessons across the curriculum that approve of same-sex marriage.
Chaplains who work in the NHS or the Armed Forces could be dismissed if they preach that marriage is just between a man and a woman.
Foster carers could be barred from looking after children if they say they disagree with allowing two people of the same sex to marry.
And religious groups could be banned from using public buildings over their support for traditional marriage.
Leading human rights lawyer Aidan O’Neill QC has written a legal opinion on the civil liberty implications if marriage is redefined.
He says that clergy could be taken to court if they refuse to conduct a gay wedding, and churches may be better protected if they stopped hosting any weddings at all.
He also says the Church of England would be less likely to face litigation if it broke off all ties with the state.
A summary of the legal opinion has been published on the website of the Coalition for Marriage, a campaign group that opposes redefinition.
The document shows the far-reaching consequences of the Government’s plan to redefine marriage ahead of the 2015 general election.
Reacting to the news, Dr Sharon James of the Coalition for Marriage said: “Mr O’Neill’s expert summary is incontrovertible proof that legalising gay marriage is not only a legislative minefield but an unparalleled attack on the freedom of conscience Britons have under the law.
“The redefinition of marriage would ride roughshod over a person’s right to support marriage as the exclusive union between one man and one woman, whether that person be a teacher, a parent, a foster carer or a marriage registrar.
“The only winners from a change in marriage law will be lawyers, who will make a fortune out of what would rapidly become a legal mess of untold proportions.
“I hope the irony is not lost on the Government that their misguided campaign to legalise gay marriage would create a society with fewer freedoms than before.”