The MP who introduced same-sex marriage legislation for Northern Ireland has urged the Secretary of State to ensure all couples will be able to get married.
Those who previously entered civil partnerships have vowed to take legal action after learning they may have to delay tying the knot under official proposals.
From January, couples who are legally unconnected will be able to get married.
Westminster voted to force the Government’s hand despite opposition from Northern Ireland’s largest party, the DUP.
Labour’s Conor McGinn introduced the successful equal marriage amendment in the House of Commons in July.
He said: “I have previously raised in the Commons my concern that, despite the will of the House to introduce equal marriage in Northern Ireland, not in any partial or segmented form, those who are in a civil partnership or those who have deeply-held religious beliefs may be forced to delay indefinitely their marriage.
“The Secretary of State (Julian Smith) should reconsider and make clear that, as of January 13, all couples in Northern Ireland who love each other will be able to get married.”
The Government is due to consult further on the issue.
Lord Hayward, who introduced one of the successful marriage equality amendments in the House of Lords, said: “It was my stated aim that equal access to marriage in Northern Ireland meant all forms of marriage – including conversion of civil partnerships and marriages with a religious ceremony where a church wishes to offer this facility.
“Throughout the debates in the Lords, I identified that I was proposing amendments to be consistent with England and Wales law and that I was working on the advice of government lawyers.
“There should be no delay in delivery and no deviation from equality for all on the same date.”
Patrick Corrigan, Northern Ireland director of Amnesty International, part of the Love Equality campaign for marriage equality, said both houses of Parliament had voted overwhelmingly in favour of extending marriage equality to same-sex couples in Northern Ireland by January 13.
He said: “It’s nonsense for the Government to now backtrack on that obligation and suggest that MPs and peers only wanted this to mean marriage for some couples, but not for others.
“We welcome the continued support of Conor McGinn and Lord Hayward for our simple demand that all people should be treated equally under the law.
“We are determined that, when the law changes come into effect in January, no-one will be left behind.”