The Christian Institute (CI) has threatened legal action against Secretary of State Julian Smith if incoming same-sex marriage laws do not protect "religious freedoms".
The campaigners, who backed Ashers Baking Company in its row over a cake with a slogan supporting same-sex marriage, claim the laws coming to Northern Ireland in January will not include provisions to protect religious freedoms and free speech as they do in England, Scotland and Wales.
However, a spokesperson for the UK Government stated that Mr Smith informed Parliament last month that a public consultation on the issue of religious same-sex marriage and protection of religious freedoms in Northern Ireland is being prepared for publication, subject to confirmation by an incoming government.
Abortion and same-sex marriage were decriminalised here after the Northern Ireland (Executive Formation) Act 2019 came into force following a vote by Westminster in July. The controversial issues were added into a Bill to keep the country running if the Executive here was not restored by October.
The CI states that same-sex marriage laws in Britain include extensive provisions to protect religious freedoms and free speech, but claim it does not appear that this will be the case here.
The legal letter from CI to Mr Smith states: "Our clients will look to challenge by way of judicial review any failure to reflect the balanced treatment of the issues in a manner reflected in the same-sex marriage legislation in England, Wales and Scotland." The CI also says religious bodies and celebrants must have protection so they cannot be forced to take part in a same-sex wedding.
CI spokesman Simon Calvert said: "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. The new law has to make this clear.
"In the rest of the UK, when same-sex marriage came in, the GB Equality and Human Rights Commission 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'."
A UK Government spokesperson stated: "Parliament passed legislation which requires the government to put in place legislation to allow for same-sex marriage and opposite sex civil partnership in Northern Ireland by January 13, 2020. We are working to meet this deadline."