The Christian Institute is warning the Government that it will take legal action if same-sex marriage is introduced in Northern Ireland without proper safeguards.
Northern Ireland Secretary Julian Smith faces being taken to court over the controversial legislation planned to come into force in January.
Lawyers acting on behalf of the Institute pointed out that same-sex marriage laws on the mainland include extensive provisions to protect religious freedoms and free speech, but these are not currently in the proposed law for Northern Ireland.
When same-sex marriage was introduced in other parts of the UK, protections were put in place to allow for conscientious objection to protect those who believe that marriage is between a man and a woman.
The Institute says religious bodies and celebrants must have protection so they cannot be forced to take part in a same-sex wedding, and that school teachers must not be compelled to promote or endorse same-sex marriage.
The legal letter, sent by Belfast solicitors Carson McDowell, states: “Our client expects same-sex marriage regulations in NI to include no less in the way of legal protections than is provided for in the rest of the UK.
“We also expect similar assurances to be given by ministers in relation to the impact on education and for similar appropriate guidance to be issued to schools.”
Simon Calvert, The Christian Institute’s Deputy Director for Public Affairs, said: “Parliament took the best part of a year to debate introducing same-sex marriage in England and Wales. For Northern Ireland, MPs took only a couple of hours.
“In the rest of the UK there are many protections for those who disagree with same-sex marriage. Not so in Northern Ireland, with the Secretary of State and the Northern Ireland Office not seeming to be bothered.
“Churches must not be sued if they refuse to do a same-sex marriage. And public order law must be amended to stop church ministers being prosecuted for sermons that disagree with same-sex marriage.”
In the rest of the UK, the Equality and Human Rights Commission has issued guidance to make clear that no school or individual teacher is under a duty to support, promote or endorse marriage of same-sex couples.
Mr Calvert said it was “essential” that the Northern Ireland Office implemented similar guidance.
He concluded: “The CI is calling for protections for public sector workers, school children, churches, religious charities and the upholding of free speech.”
The Institute’s call for proper safeguards has drawn support from pro-same-sex marriage groups.
LGBT campaigner Peter Tatchell told the News Letter: “The law in the north of Ireland should be the same as the rest of the UK.”
Rainbow Project Director John O’Doherty and Patrick Corrigan of Amnesty International also agreed that the same protections and freedoms available in England and Wales should be available in Northern Ireland.