The Lords’ vote on the same-sex marriage Bill has been moved by the Government from Monday 3 June to Tuesday 4 June.
The timing has been changed because the Government feared it could lose a vote on the first day back after recess.
The debate will still start on Monday, and continue into Tuesday with a vote expected on Tuesday evening.
The Coalition for Marriage is urging the Lords to vote against the Bill. Campaign director, Colin Hart, said: “The Government is clearly worried that we have a lot of votes on our side, and that the Bill is at risk of defeat. That is what the media are saying.
“We have heard from Peers in all parties that their mailbags are overflowing with people who want marriage to remain as it is.”
He added: “If Peers vote against the Bill on Tuesday, the Bill is defeated outright. But if that fails, there is still all to play for.
“There will be other votes in the Lords. A win for us would be decisive, but a defeat still keeps us in the game.”
In the Lords opposition to the Bill is being led by independent Peer, Lord Dear. He thinks the result is too close to call.
He said: “This is ill-thought through legislation that is being rushed through”.
“There are some 8,000 further amendments that will be necessary to existing legislation because of this single policy.
“Of those who said they would speak about half seem to be opposed. I really think the vote will be too close to call.”
Other Peers have also expressed concern, with Lord Luce, who served as a minister in Margaret Thatcher’s government, saying the issue has been handled in a “careless manner” with “little consultation” and “little thought”.
Lord Stoddart, an independent Labour Peer, said the concept of gay marriage is “bogus”, and that many homosexual people “do not want this”.
Earlier this month Prime Minister David Cameron admitted that the country is divided on gay marriage.
David Cameron said he will stop focussing on such policies and concentrate on “big picture” issues.
Speaking to BBC Radio 4, Mr Cameron said, “if you’re saying to me, ‘Is this the first now of many other issues like that?’ No it isn’t. The Government now is going to be absolutely focused on the big picture, which is fixing our economy, reforming welfare, making sure there are good schools for our children to go to”.