The first gay weddings are set to take place in England and Wales in the summer of 2014, after controversial legislation to introduce same-sex marriage cleared the House of Lords.
Jubilant gay rights campaigners vowed to press for equal marriage in Scotland and Northern Ireland, as they celebrated the successful passage of the Marriage (Same Sex Couples) Bill through the Upper House.
With the final debates in the Commons expected to be little more than a formality on Tuesday, the Bill is likely to become law within days.
As dozens of supporters wore pink carnations in the Lords to see the Bill receive an unopposed third reading, openly gay Labour peer Lord Alli thanked peers in an emotional speech, declaring: "My life and many others will be better today than it was yesterday."
And Deputy Prime Minister Nick Clegg told campaigners celebrating outside parliament that the new law would ensure that lesbian, gay, bisexual and transgender (LGBT) people felt "recognised and valued, not excluded".
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However, opponents of gay marriage warned that the controversial government-backed reform would "come back to bite" Prime Minister David Cameron.
The Coalition for Marriage campaign group said it would mobilise a 700,000-strong support base in next year's European elections and the general election of 2015.
The Bill survived a stormy passage through the House of Commons despite fierce opposition from dozens of Tory backbenchers, and then overcame an attempt to "wreck" it in the House of Lords last month.
Deep divisions over the legislation persisted right to the closing moments of the Lords debate.
