John Major has told those opposed to same-sex marriage to "move on" and accept that gay people should have the right to marry, as at least 100 Tory MPs prepare to vote against the measure.
The former Conservative prime minister said on Monday that David Cameron's pledge to allow gay people to get married in the face of hostility from a large number of his own MPs was "courageous".
“We live in the 21st Century and must move on: every couple should have the opportunity and the right to formalise their relationship," Sir John said.
"The prime minister’s instinct to support equal marriage is a courageous and genuine attempt to offer security and comfort to people who – at present – may be together, yet feel apart."
On Friday Cameron tackeld opponents of gay marriage head on by announcing churches would be able to conduct same-sex weddings if they wanted to.
Previously it had been expected that the government would legislate to specifically ban religious premises from holding weddings, in order to appease opponents who fear churches will be forced to conduct them against their will.
In his high profile intervention today, Sir John said: "I fully understand that there are many who will find this difficult to accept, as will the Churches.
“But the prime minister has made it clear that the Churches will be free to make their own decisions upon whether to conduct such marriages - and that is entirely the right approach."
Culture secretary Maria Miller is due to formally respond to the government's consultation on gay marriage on Wednesday, with a Bill expected to be introduced in the Commons soon after.
At least 100 Tory MPs are likely to vote against the legislation, with Stewart Jackson telling HuffPost UK last week that the plan was a "recipe for disaster and division in the Conservative Party".
Harrow MP Bob Blackman did little to help the prime minister's goal of detoxifying the party on the issue by calling for the re-introduction of Section 28, the Thatcher-era legislation that banned teachers from talking about homosexuality.
And fellow Tory David Davies, the Monmouth MP, told BBC Wales on Sunday that "most parents would prefer their children not to be gay".
On Sunday senior Conservatives including Boris Johnson and Michael Gove launched a new group to campaign group to argue in favour of gay marriage.
Johnson said the Government should get on and "whack through" the gay marriage legislation. "It is perfectly obvious that the constituency is there for doing this," he told Sky News' Murnaghan programme.
And on Monday Conservative MP Gavin Barwell said telling same sex couples they did not need to get married was not so different from when black people in the Deep South of the United States used to be told “You don’t need to sit in this part of the bus, you’ve got your own seats at the back”.
Writing on his blog he said: "For too long our Party has allowed itself to be portrayed as against people - against immigrants, against single mums, against gay and lesbian people. It seems to me that a Conservative Party that stands up for religious freedom but is also for those who love and want to make a public commitment to another human being of the same sex and for a society in which everyone is treated equally under the law would be best placed to win an overall majority at the next Election - and would deserve to do so."