The impending gay marriage legislation could force Queen Elizabeth II to break a sovereign promise, a former senior UK bishop has warned.
Bishop Michael Nazir-Ali, who was Bishop of Rochester for 15 years, said the Queen took an oath to "uphold God's laws" when she was crowned.
But the signing into law of a bill currently being debated in the House of Lords to allow same-sex couples to marry would force her to break that promise she made upon her coronation.
Peers begin two days of debates on the government's plans to allow same-sex weddings on Monday, with a vote taking place on Tuesday.
"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," said Nazir-Ali, who called on Prime Minister David Cameron to not put the 87-year-old monarch in such a difficult position.
"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. But while she is faithful to the vows she has taken, in this country she has to act on the advice of her ministers.
"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," he said.
The government's bill has exposed deep divisions within the Conservative party on the issue when it was passed in the House of Commons last month amid opposition from 133 Tory MPs.
The Marriage (Same Sex Couples) Bill for England and Wales would allow couples, who can currently form civil partnerships the rights to marry.
Religious organisations would have to "opt in" to offering weddings, with the Church of England and Church in Wales being banned in law from doing so.
Some 86 peers are expected to speak in today's debate with a final vote due on Tuesday
Lord Alli, a staunch supporter of the bill, said the government could force through legislation using the Parliament Act even if peers in the Lords vote to reject the bill.
However, the act is rarely used and could force a "constitutional crisis", he warned.
The Conservative Grassroots group has called on peers to reject the bill and in a letter to the Sunday Telegraph stressed on the "negative effect" it was having on the party.
"It is alienating much of our core support while failing to attract new voters with under-two years to go before the general election," chairman Robert Woollard wrote.