Registration opens for first same-sex marriages
Same-sex couples in England and Wales who want to hold weddings on the day gay marriage becomes legal on 29 March can register to do so from Thursday.
Existing marriages of gay couples that took place overseas will also be legally recognised from Thursday.
Procedures for people who are in civil partnerships to "upgrade" to marriage have yet to come into effect.
Scotland has passed a same-sex marriage bill but changes in the law are not being introduced in Northern Ireland.
'Toast to equality'
Couples have to give 15 days' notice of their intention to marry at a register office, and need to do so on Thursday if they want to be among the first to benefit from the passing of the Marriage (Same-Sex Couples) Act.
Under the terms of the bill, religious organisations will have to "opt in" to offering weddings, with the Church of England and Church in Wales prevented in law from doing so.
One of the first couples affected by the legislation on overseas marriages said they had been "determined" their union be acknowledged in their home country.
Celia Kitzinger, 57, and Sue Wilkinson, 60, were married in Canada in 2003 and previously lost a legal fight over the issue.
The couple were told by a High Court judge in 2006 that their marriage was not valid in England and that they faced an "insurmountable hurdle" for it to be declared as such.
But the changes introduced in the same-sex marriage act passed in Parliament last July meant that it has now automatically been made legally binding.
Sociology professor Ms Kitzinger, who works at the University of York, said: "I came out as lesbian in 1973, and things were a lot different then, and it never occurred to me that one day the law would be changed such that my marriage to the woman that I love would be recognised.
"We're the 15th country in the world, so it's been a bit slow and behind countries you would be quite surprised about."
Speaking ahead of the law change, she said she and her wife would be celebrating at home in Gribthorpe, East Yorkshire, toasting "the future of lesbian and gay equality for all countries of the world".
Ms Wilkinson, a professor of social sciences at Loughborough University, said the couple may have opted for a civil partnership were it not for the fact she was based in Canada when the law changed there.
"But having had marriage and experienced what that felt like made us determined it had to be marriage and nothing but marriage," she added.
Civil partnerships became legal in the UK in 2004, in effect giving same-sex couples the same legal recognition as heterosexual marriage.
Those in civil partnerships can choose to convert their relationships to marriage but are under no obligation to do so.
The procedure for such a conversion is expected to be in place by the end of the year.
The notice period to register intent to marry can be waived in the case of a same-sex couple where one of the partners is terminally ill, Pink News reported.
Ceremonies in Scotland are expected to take place before the end of the year.