Legislation to introduce same-sex marriage is expected to complete its passage through the House of Commons, paving the way for the first gay weddings in England and Wales next summer.
On Monday night, jubilant gay rights campaigners were celebrating the successful passage of the Marriage (Same Sex Couples) Bill through the House of Lords - and vowing to take the fight for marriage equality to Scotland and Northern Ireland.
MPs are to debate amendments made to the legislation in the Upper House, but this is expected to be little more than a formality and is unlikely to delay its progress into law, with Royal Assent within the next few days.
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".
And openly gay Labour peer Lord Alli told fellow members of the second chamber - some of them sporting pink carnations: "My life and many others will be better today than it was yesterday."
But 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 its support base in next year's European elections and the general election of 2015.
Coalition chairman Colin Hart said: "Mr Cameron needs to remember that the Coalition for Marriage has nearly 700,000 supporters, nearly six times the number of members of the Conservative Party. They are just ordinary men and women, not part of the ruling elite. They are passionate, motivated and determined to fight on against a law that renders terms like husband and wife meaningless and threatens one of the foundations of the institution of marriage: fidelity and faithfulness."
The Bill survived a stormy passage through the Commons despite fierce opposition from dozens of Tory backbenchers, and then overcame an attempt to "wreck" it in the House of Lords last month.
Stonewall chief executive Ben Summerskill said: 'It's impossible to express how much joy this historic step will bring to tens of thousands of gay people and their families and friends. The Bill's progress through Parliament shows that, at last, the majority of politicians in both Houses understand the public's support for equality - though it's also reminded us that gay people still have powerful opponents."
Mr Clegg said the Bill was a mark of "the kind of open, modern, tolerant and diverse society we want Britain to be in the 21st Century". Writing on the PinkNews website, the Liberal Democrat leader said: "Civil partnerships were a landmark reform. And not every couple, gay or straight, feels that they need to get married to affirm their commitment to each other. But only the right to marry, if that's what you so choose, is true equality. It lets every member - young or old - of our LGBT community know that they are recognised and valued, not excluded. It finally ensures that all loving couples have the freedom and right to make that commitment to each other in our society."