Peers have begun debating the Marriage (Same Sex Couples) Bill in the House of Lords as part of its second reading.
More than 90 peers have requested to speak in the debate, which will continue on Tuesday.
A final vote by peers is expected at 6pm tomorrow.
Earlier, human rights campaigner Peter Tatchell said to PinkNews.co.uk: “It is uncertain how the House of Lords will vote. We don’t know the extent of the opposition. It would be a big mistake to assume that equal marriage is a done deal.”
The former Chief Constable of West Midlands Police, Lord Dear, has tabled a ‘fatal’ amendment to deny the bill its second reading.
“Yes there will be some people who oppose it just as they have opposed previous legislation”, she said.
“Yes, some opposition speeches will be vociferous, and yes some will be a little bizarre. But we also know the votes to support this bill are there, in good numbers.”