Gay marriage advocates have warned that the same-sex marriage Bill could be put at risk by a late night vote in the House of Lords, as the peer leading opposition to the legislation says the majority are "affronted" by the plans.
At last count 80 peers had asked to speak in the debate which begins at 3pm on Monday - meaning it could last well into the early hours of Tuesday morning.
Labour has warned a 2am vote could be "very risky" as frail and elderly peers, as well as those who live far away from London, may not be able to turn up to vote in favour of the Bill.
Lord Dear, the cross-bench peer who is leading opposition to the Bill, said he expected the vote to be much closer than in the Commons where it received overwhelmingly support. "I don't think it will be a landslide either way," he told The Huffington Post UK.
"The one thing I can be sure of there is a great deal of dissatisfaction with an ill thought though Bill."
The government Whips office has insisted it will do "nothing to jeopardise" the equal marriage legislation - pursuit of which has split the Conservative Party down the middle.
But Labour shadow equalities minister Baroness Thornton said the debate should instead be stopped before midnight on Monday and then started up again on Tuesday morning.
She told HuffPost UK: "There are a significant number of peers who really want to support this vote but are not strong enough to stay until 3am in the morning. It's wrong to vote on something really important in the early hours of the morning."
Baroness Thornton said Cameron had been "absolutely brilliant" in his advocacy of gay marriage and urged him to change the timing of the vote. "We are worried. It's a bit irresponsible. Those very opposed will stay to the bitter end," she said.
Lord Dear, a former chief constable of West Midlands Police, said he did not think the proper ground work had been done on the Bill before being introduced in the Commons. "There was no Green Paper, no pre-legislative scrutiny, it wasn't in the Queen Speech," he said.
He said opposition to the Bill on procedural grounds was "mixed up in a potpourri of attitude of saying we shouldn't be doing it at all".
"I am in favour of the status quo," he said."If I could be assured the whole revision of the state of marriage had been properly thought through, I would be, if not happy, very content, to go the other way."
Anti-gay marriage campaigners have also argued that the convention that stops the Lords shooting down legislation passed by the Commons does not apply as same-sex marriage was not explicitly included in either the Conservative or Lib Dem election manifestos.
Tory backbencher Peter Bone has said peers would have "complete legitimacy" if they chose to reject the Bill.
Lord Dear said that while peers would "only in extremes" throw out a government bill, the rule was "not set in concrete" - especially given the Commons passed the Bill on a free vote.
He told HuffPost UK while it was the mark of a "civilized society" that minorities be protected, in this case "the minority is so small and the majority is so affronted" that the government should reconsider.
"There is a great deal of dissatisfaction with an ill-thought though Bill," he said. "Do we know exactly where this will all lead us?"
"I think there is enormous concern about why have we done it now. Why has it been pushed thorugh the Commons in the way that it has with insufficient research, where it is going to lead us and what is it going to do to the institution of marriage as we know it," he said.
The latest YouGov poll showed that the public backed gay marriage by 54% to 37%. The survey conducted earlier in May also showed there was also a huge generation gap. Under 40s backed the Bill by more than three-to-one byt voters over 60 oppose it by more than two-to-one.
The Commons easily passed the Bill earlier this month despite more Tories voting against it than in favour, thanks to the votes of Labour and Lib Dem MPs.
MPs were given a free vote, but Tory backbenchers have complained that they were threatened by party whips to either back the Bill or see their future careers ruined.
Nick Herbert, Cameron's former police minister who has led the backbench pro-gay marriage contingent, has urged peers to respect the will of the elected Commons.
"The Lords always have important scrutiny job, but can't ignore fact that this Bill passed the elected House with a 2:1 cross-party majority," he said.
Peers are also being encouraged to support the bill by the LGBTory group within the Conservative Party. The organisation has sent letters to members of the Lords arguing that "supporting equal marriage is the right thing to do: popular with most people and recognises all citizens as equally valid".
The letter adds: "We are proud that it is a Conservative prime minister who is leading on equal treatment for all and we believe the government has been thoughtful and appropriate with this legislation and so got it right."