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Gay marriage could be set in statute by the end of the week, with the first gay weddings expected in England and Wales next summer.
Equality campaigners celebrated outside Parliament after the Marriage (Same Sex Couples) Bill was passed successfully through the House of Lords on Monday, ahead of this evening's final Commons debate which has been described as a formality.
Once the bill has passed both houses, the Queen will be bound to give it the Royal Assent - bringing to an end centuries of tradition in which marriage was a contract between a man and a woman.
Speaking outside Parliament, Deputy Prime Minister Nick Clegg told a jubilant crowd that the law would ensure that lesbian, gay, bisexual and transgender people felt "recognised and valued, not excluded".
In an emotional debate in the Lords, Labour peer Lord Alli, who is gay, said it had been "truly humbling" to play a role in implementing the reform.
"As a gay man over those 15 years you [the House of Lords] have changed my life. You have given me dignity where there was sometimes fear. You have given me hope where there was often darkness and you have given me equality where there was sometimes prejudice.
"This is a special place and I am proud to have figured in it. My life and many others will be better today than it was yesterday," he told fellow peers, many of them wearing pink carnations.
Baroness Stowell, the government spokesman who steered the bill through the Lords, called it an "historic" achievement.
However, opponents warned the controversial reform would "come back to bite" Prime Minister David Cameron.
They accused the Government of using a parliamentary "bulldozer" to speed the change through, following one of the most acrimonious debates of recent history, which divided the Conservative party and drove a wedge between Church and State.
The Coalition for Marriage campaign group said it had built up a database of around 700,000 supporters, and would mobilise that backing to target marginal seats in next year's European elections and the general election of 2015.
Colin Hart, campaign director of C4M, 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 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 fierce opposition from dozens of Tory backbenchers to pass through the Commons, and then overcame a "wrecking" bid in the Lords last month.
James-J Walsh, campaigns director of Out4Marriage, hailed the Lords' approval of the Bill last night as "a momentous occasion in lesbian and gay equality in England and Wales, overcoming the last major hurdle on our path to full legislative equality.
"With this legislation Britain shines a light as one of the most progressive countries in the world, and manages to balance both lesbian and gay rights with religious freedoms."
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."