Ministers are braced for a new battle over plans to allow gay marriage as the legislation begins what is expected to be a bruising passage through the House of Lords.
Peers from all parties are set to unite in a bid to derail the controversial move, with David Cameron under pressure from Tory activists to abandon the reform.
But he was bolstered by the support of several senior Conservative figures who called on colleagues not to "hinder a measure whose time has come".
Six former ministers - including five veterans of Margaret Thatcher's cabinets - argued in a letter to The Times that the allowing same-sex marriage would strengthen the institution.
Some 86 members of the upper house have asked to speak in the second reading debate on the Marriage (Same Sex Couples) Bill, forcing the Government to delay a vote to avoid a potential defeat.
It was postponed to later tomorrow amid fears opponents were more likely to stay on into the early hours in the hope of killing off the Bill.
Cameron has personally championed same-sex marriage in the face of vehement opposition from many in his own party and church leaders.
More Conservative MPs voted against it than for in the Commons, but Labour and Liberal Democrat support meant it was eventually passed by a majority of 205 in a free vote following a highly-charged debate.
The Tory leader has made efforts to calm tensions with party members over the issue but remains under serious pressure to make a U-turn.
In a rival letter to the newspaper, a number of former and serving local party chiefs launched a fierce attack on what they said was a "politically toxic" measure that was driving voters away from the party.
Led by Robert Woollard, chairman of the Conservative Grassroots campaign which is spearheading criticism of Cameron's leadership, they said Tory peers had "every reason to oppose this Bill and vote it down.
"The central precept of the Bill stands in stark contradiction to the central Conservative commitment to marriage, family and children," they wrote, attacking the "aggressive and manipulative" way it was being pushed through parliament.
"This Bill is politically toxic for the Conservative Party. Not only has it alienated the grassroots activists from the leadership but it is driving our traditional voters elsewhere while failing to draw in many new ones."
The pro-same-sex marriage letter was signed by Tory former cabinet ministers Lord Fowler, Lord Jenkin of Roding, Lord Hunt of Wirral, Lord Deben and Baroness Bottomley of Nettleston as well as ex-minister Lord Garel-Jones.
They argued that there was a public majority in favour and pointed to similar reforms around the world. France became the latest country to legalise same-sex marriage last week in the face of some violent protests.
"We believe it is right to open up marriage to loving and committed same sex couples, and that this important institution will be strengthened by the change," they wrote.
"The Bill rightly enshrines the principle of religious freedom, protecting those faith groups that do not wish to conduct same-sex marriages, but allowing others to do so if they wish.
"A majority of the public now supports same-sex marriage, and legislatures around the world are reflecting this change of attitude. The elected House of Commons passed this Bill on a free vote by more than a two to one cross-party majority.
"The House of Lords should consider this legislation carefully, but it would be wrong to hinder a measure whose time has come."