The Queen would be breaking her oath of coronation if she signed gay marriage in to law, according to a former leading Bishop.
Bishop Michael Nazir-Ali said the Queen had a duty to "uphold the laws of God" and that this was at odds with allowing gay couples to wed.
The former Bishop of Rochester made the comments during a service outside Buckingham Palace on Sunday, ahead of a key vote on gay marriage in the House of Lords this week.
He later told the Daily Mail: "The idea of a constitutional monarchy comes from the Bible. Christians are told in the Bible to obey their rulers, unless the ruler tells us to do something God forbids. Happily in this country we have a monarchy that has taken an oath of upholding God’s laws, and the present Queen has for years been faithful to that. We are praying that she continues to be faithful.
"That puts the onus on the prime minister not to put the Queen into a position where she may have to go against the sovereign promises she has made. We hope that she is not put in that position."
Nazir-Ali caused controversy in 2009 when he said while gay people were welcome to attend church they should "repent and be changed."
During her oath in 1953 the Queen pledged to "maintain the Laws of God". On Tuesday, she will attend a service at Westminster Abbey to mark 60 years since the coronation. On the same day peers will vote on whether to allow David Cameron's gay marriage Bill to continue its passage through parliament.
According to the Daily Telegraph, senior Church of England officials have advised Bishops in the Lords to stay away from the vote. It has been reported the Church fears the sight of unelected religious leaders voting against a same-sex marriage Bill that was overwhelmingly passed by the Commons could re-open the debate over whether it is right Bishops get to sit in parliament.
Peers from all parties are set to unite in a bid to derail the gay marriage, with Cameron under pressure from Tory activists to abandon the reform.
But he was bolstered by the support of several senior Conservative figures who called on colleagues not to "hinder a measure whose time has come". Six former ministers - including five veterans of Margaret Thatcher's cabinets - argued in a letter to The Times that the allowing same-sex marriage would strengthen the institution.
Some 86 members of the upper house have asked to speak in the second reading debate on the Marriage (Same Sex Couples) Bill, forcing the Government to delay a vote to avoid a potential defeat.
It was postponed to later tomorrow amid fears opponents were more likely to stay on into the early hours in the hope of killing off the Bill.
Cameron has personally championed same-sex marriage in the face of vehement opposition from many in his own party and church leaders.
More Conservative MPs voted against it than for in the Commons, but Labour and Liberal Democrat support meant it was eventually passed by a majority of 205 in a free vote following a highly-charged debate.
Supporters of marriage equality will rally outside the House of Lords from 6pm tonight, as peers debate the Bill.
Gay rights campaigner Peter Tatchell said: "The religious and political opponents of same-sex marriage are an increasingly small and shrill minority who are out of touch with the tolerant, liberal-minded majority. Their stand against marriage equality fuels homophobia and gives comfort to bigots everywhere."