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.
Bishop Welby is being formally confirmed in his new role at a ceremony in St Paul's Cathedral on Monday.
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.
Tories have been plunged into deep unrest by same-sex marriage 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 tomorrow. He is expected to attempt to talk to his MPs today in the hope of winning their support, according to The Times.
Tory activists claimed yesterday 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."
Former prime minister Gordon Brown said: "I understand the strong feelings in the current debate but I take the view that it is now timely to agree to end another source of discrimination by legalising the right to marriage and I will support the legislation in the UK Parliament and when it comes to the Scottish Parliament."
The Marriage (Same Sex Couples) Bill, which will also allow civil partners to convert their partnership to a marriage and enable married people to change their legal gender without having to end their union, has its second reading tomorrow.
Mr Cameron is giving his MPs a free vote on the Bill, which will avoid a technical rebellion, though the high numbers
expected to oppose or abstain from within his party will still prove an embarrassment for the Prime Minister.
Strong Liberal Democrat and Labour support means the Bill, however, will comfortably reach its next stage.
Culture minister Ed Vaizey insisted the vote would not tear the Tory party apart, and claimed internal divisions over the issue were "good-natured".
Former children's minister Tim Loughton warned the Bill was "full of pitfalls" and would "set MP against MP".
Foreign Secretary William Hague said he became a supporter of gay marriage "over the last couple of years" and suggested the change would become quickly accepted by the public.