SINN Féin negotiator Conor Murphy has claimed the British government gave assurances last year that if the assembly failed to support same-sex marriage then Westminster would pass the necessary legislation.
The claim came as the two governments launched a fresh round of talks aimed at restoring devolution.
A draft agreement was rejected by the DUP in February last year amid misgivings by party representatives.
It is understood that measures aimed at introducing same-sex marriage played no part in that proposed deal.
After the talks broke down Sinn Féin faced criticism from the LGBT+ community for not making marriage equality a red line.
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Mr Murphy said under the draft deal "the issue of equal marriage was going to be presented in the assembly".
Efforts to change the law have previously been thwarted by the petition of concern – the veto designed to protect minorities in the assembly – and while the DUP alone no longer has the 30 MLAs required to trigger the mechanism, representatives from other parties opposed to same-sex marriage could sign it.
Mr Murphy claimed the Ulster Unionists could not give a commitment that none of its MLAs would do so but the British government gave assurances that Westminster could introduce the necessary legislation.
"So it could have been passed in the assembly, but had it not, we had a commitment that it would be allowed to proceed through the Westminster route," he told the BBC.
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The Sinn Féin negotiator said that as a co-guarantor of the Good Friday Agreement, the British government, working along with the Dublin government through the intergovernmental conference, had a responsibility to "provide for the rights of people here".
“And if that is done in Westminster then so be it; that is their responsibility,” he said.
A British government spokesman neither confirmed or denied Mr Murphy's claim but said it believed it was best for the north's politicians make decisions that affect people's lives in the region.