By Alex Stevenson and Ian Dunt
Senior Conservatives will have to back the coalition's timetabling plans for its gay marriage bill or resign, in a move which will encourage dissent on the government's backbenches.
While No 10 has conceded a free vote on the bill itself, which it claims is a matter of conscience, there will be a three-line whip on the programme motion.
This usually takes place after the bill's second reading and outlines the scheduling for its remaining progress through parliament.
"It's hardly a free vote, is it? If a PPS votes against it, they'll have to resign," a leading opponent of the move told politics.co.uk, ahead of next week's vote.
The comments come as Cabinet ministers start to take their positions for the battle to come.
Prominent right-wingers Chris Grayling and Sayeeda Warsi today confirmed they would support the measure.
Grayling found himself in hot water in 2010 when he said owners of B&Bs had the right to stop gay couples staying with them. Warsi is well-known for her strong religious views and was expected to oppose the measure.
Their decision to support the bill suggests Downing Street is succeeding in minimising a split in the Cabinet over the controversial bill.
The development comes as Philip Hammond caused outrage by citing the law against incest in an argument against gay marriage.
The defence secretary was accused of mentioning the ban on marriage between siblings during a discussion about same-sex partnerships by the website pinknews.co.uk earlier this week, but a spokesman denied the report.
Asked by 5Live Breakfast if he told gay campaigners that "we don't allow siblings to get married either" during a debate on gay marriage, Hammond admitted that he did.
"The context was we were having a very wide ranging discussion about the state and the state's involvement in people’s relationships and people's day to day lives, and also talking about the tax advantages that are available through marriage and civil partnership," he said.
Pressed on whether he was comparing gay marriage to incest, Hammond replied: "No, I was not. And that’s a completely scurrilous interpretation of what I said."
The comparison is still far short of some reports, which said the defence secretary said allowing gay marriage would be like "sanctioning incest".
Joe Rayment, a student at the scene, told the Guardian: "When I asked Philip Hammond what right the state has to tell two people in love that they can't get married, he said: 'Well, siblings can't get married either'. We found this a very offensive and quite disgusting thing for him to say."
Hammond is a leading gay marriage opponent in Cabinet.
The story comes as the French National Assembly starts a marathon debate on legalising gay marriage, following months of heated debate.
The introduction of gay marriage was one of Francois Hollande's election promises.