“The biggest uncertainty facing the same-sex marriage bill is how the House of Lords will vote,” Peter Tatchell, human rights campaigner and co-ordinator of the Equal Love campaign has warned.

“We just don’t know how much opposition there will be in the upper house. It would be a big mistake to assume that equal marriage is a done deal.

“There is no room for complacency,” Mr Tatchell said. “We need to lobby the Lords just like we lobbied the Commons. Church leaders and right-wing peers are determined to derail the Marriage (Same Sex Couples) Bill. They’ll try to vote it down or introduce wrecking amendments.”

Last month, Lord Dear, the former chief constable of the West Midlands, predicted that the bill could be defeated by peers, although most commentators expect it to be passed.

Mr Tatchell stressed: “While it would be unusual and unethical for the unelected Lords to overturn a bill passed by a huge 225-vote majority in the Commons, it is not impossible or unlawful.

“In the event of defeat in the Lords, it is not clear whether the government would invoke the Parliament Act to secure the bill’s passage.”

Mr Tatchell is encouraging the public to lobby wavering peers by writing to them directly.  

With the Commons committee stage of the Marriage (Same Sex Couples) Bill now complete, the joint Committee on Human Rights has been scrutinising the bill this week.

It has received submissions from Baroness O’Neill, the chair of the Equality and Human Rights Commission and Professor Christopher McCrudden, of Blackstone Chambers, the lawyer who advised the Catholic Church on the bill.

The House of Lords will likely debate the reform next month, although dates have not yet been announced.