Cameron faces a setback in the lower chamber, the House of Commons, if the opposition Labour Party joins forces with Tory rebels in a vote on his bill.
While Cameron, several senior party colleagues and his Liberal-Democrat coalition partners back the bill legalising gay marriage, large sections of his Conservative party are fiercely opposed to the idea.
Dozens of disgruntled lawmakers are expected to deliver heavy blow to Cameron's agenda by backing an amendment Monday calling for civil partnerships to be offered to heterosexual couples.
Downing Street called it a "wrecking amendment" which could cost 4 billion pounds (6 billion dollars) in pension changes and delay the passing of the same-sex marriage bill by up to two years.
Labour however, while it backs gay marriage, is set to join the rebels, arguing that the government has not made a convincing case against extending civil partnerships.
France on Saturday became the 14th country to legalise gay marriage when Socialist President Francois Hollande signed it into law. The measures were bitterly opposed by France's main opposition right-wing UMP party.
In Britain however, the ruling Conservative party appears to be at war with itself over the issue.
A letter signed by more than 30 current and former Conservative association chairmen, handed to Cameron on Sunday, gave an indication of the bitterness felt by some sections of the party.
They accused him of "treating the membership with contempt" over the issue.
"You have failed thus far to keep the manifesto promise you made to recognise and incentivise marriage through a tax break for married couples," it said.
The vote comes just days after a member of Cameron's inner circle fiercely denied claims that he had branded grassroot party members as "mad, swivel-eyed loons".
Party co-chairman Andrew Feldman has strenuously denied Internet rumours linking him with the statement and said he was considering legal action.
Cameron is already under pressure from the right-wing of his party over Europe.
Senior members of the Conservative Party are concerned about the rise of the anti-European Union UK Independence Party (UKIP) after its unprecedented gains in recent local elections.
That threat prompted around 114 Tory MPs last week to back a motion in parliament expressing regret that the coalition government's plans for the year contained no guarantee of a referendum on EU membership.
UKIP leader Nigel Farage has placed a full-page advertisement in Monday's Daily Telegraph newspaper, urging more Tory voters to switch allegiances. Seizing on the alleged "loons" comments, he described them as "the ultimate insult" from party leaders.
"Only an administration run by a bunch of college kids, none of whom have ever had a proper job in their lives, could so arrogantly write off their own supporters," he wrote.
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