The former Bishop of Rochester says introducing equal marriage could force the Queen to break a sovereign promise.
Dr Michael Nazir Ali said the Queen vowed when she was crowned 60 years ago to “uphold the laws of God”.
The 63-year-old Anglican made the comments yesterday in a service outside Buckingham Palace on the 60th anniversary of the Coronation.
He later told the 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.”
The Marriage (Same Sex Couples) Bill will receive its second reading in the House of Lords on Monday afternoon and peers will vote on the bill tomorrow evening.
It declared: “We are implacably opposed to all forms of discrimination, whether rooted in gender, race, colour, creed, political belief or other grounds.”
The words “other grounds” were interpreted by some as including sexuality.
While Ben Summerskill, chief executive of Stonewall, said the Queen had taken “an historic step forward” on gay rights, several other LGBT activists including the campaign group Justice for Gay Africans dismissed the idea that the 87-year-old monarch had suddenly adopted a new position on LGBT equality.