The new Archbishop of Canterbury is ready to reveal he believes that marriage should be between a man and a woman.
Ahead of the first parliamentary vote on the reforms, the Rt Rev Justin Welby is prepared to face questions about the highly divisive issue.
Tories have been plunged into deep unrest by the proposals, which David Cameron has personally championed.
The Prime Minister is facing the prospect of some 180 members of his party, including a significant number of senior figures, opposing or abstaining in a vote on the changes on Tuesday. He is expected to attempt to talk to his MPs in the hope of winning their support, according to The Times.
Bishop Welby is set to be formally confirmed in his new role at a ceremony in St Paul's Cathedral. A source told the Daily Telegraph: "He will say that marriage is between a man and a woman, and always has been."
Lambeth Palace was keen to stress the view was standard Church of England policy and insisted the Archbishop was not planning to wade into the row by making any formal statements, but was simply ready to respond to any questions he was asked on the issue.
Tory activists claimed on Sunday they felt "a sense of betrayal" over the Prime Minister's "bulldozed-through" reforms and handed in a letter to No 10 urging Mr Cameron to rethink the plans.
Geoffrey Vero, chairman of the Conservative association in Surrey Heath where Education Secretary Michael Gove is MP, warned the move "may seriously affect David's opportunity to get re-elected at 2015".
But human rights campaigner Peter Tatchell said Tory opponents did not represent most Conservative supporters. He added: "Opposition to equal marriage by some Conservatives is reviving the 'nasty party' image and turning off voters. It undermines David Cameron's attempt to detoxify the Tory brand and present a more caring, compassionate Conservatism.
"The Prime Minister's backing for marriage equality is making many voters more sympathetic to the Conservatives. The anti-gay marriage push by backbench rebels is likely to drive crucial centre-ground swing voters away. They will decide the next election, not anti-gay Conservative MPs and constituency associations."
UK 
