A Conservative cabinet minister has declared his opposition to same-sex marriage in the most significant challenge yet to reform plans backed by the prime minister.
Northern Ireland secretary Owen Paterson made clear his hostility to the change in a letter to a constituent, seen by the PoliticsHome website.
"Having considered this matter carefully, I am afraid I have come to the decision not to support gay marriage," he wrote in response to an appeal for support.
David Cameron has personally championed the change, which is out for public consultation, but is under pressure from backbench Tories to drop the move.
Defence secretary Philip Hammond said recently that it was "not a priority" for the government after the Liberal Democrat policy was blamed by some for poor local election results.
His Cabinet colleague went further, expressing direct opposition.
In a letter dated last week, Mr Paterson said that he backed "advancing equality" a previous move to allow religious premises to carry out civil partnerships if they wished.
"The prime minister has made clear that he supports equal civil marriage and the government is rightly consulting widely on this issue before making any changes," he added.
Andrew Smith, the North Shropshire constituent whose appeal for support the senior Tory rejected, said his MP's stance was illogical.
"It is bizarre that a cabinet minister can praise his government's progress on LGBT equality in the same letter that he tells me he won't support my right to marriage equality.
"He is clearly unsympathetic to the needs that LGBT youth in rural communities have for leaders who stand up for equality," he told PoliticsHome.
A Number 10 spokesman would not comment directly on Mr Paterson's stated opposition but said the government remained committed to the reform and to passing the necessary legislation before the end of the Parliament.