Hundreds of gay couples are preparing to form civil partnerships in the coming weeks as the law changes after decades of campaigning.
Brian Jones (left) and Anthony Carter register their intent to form a civil partnership
At least 1,200 ceremonies are confirmed as being scheduled already, according to figures from councils compiled by the BBC News website.
Registrars are preparing for the first ceremonies, with couples permitted to register from Monday morning.
Campaigners say the law ends inequalities for same-sex couples.
The first ceremonies under the Civil Partnerships Act can take place in Northern Ireland on 19 December, followed by Scotland the next day and England and Wales on 21 December.
Under the law, couples who want to form a partnership must register their intentions with local councils. Unlike marriages, the signing of the legal partnership papers does not need to happen in public.
Bookings coming in
Hundreds of couples are expected to go ahead quickly, with Brighton conducting 198 ceremonies before the end of the year. Overall, the city has taken 510 bookings for the coming months, thought to be the highest in the country.
Other cities which have seen strong interest include London, Manchester, Birmingham, Newcastle and Edinburgh.
Brighton and Hove: 510
Source: Councils; not all ceremonies booked for December
Meg Munn, minister for equality, said the government expected 4,500 couples to get "partnered" in the first year.
"This is an important piece of legislation that gives legal recognition to relationships which until now were invisible in the eyes of the law," Ms Munn told the BBC News website.
"It accords people in same-sex relationships the same sort of rights and responsibilities that are available to married couples.
"We know there are people who have been together maybe 40 years and have been waiting for the chance to do this kind of thing, because of the important differences it makes to their lives.
"They have the same concerns as married couples - tenancy, ownership, pensions and inheritance."
Alan Wardle, of gay campaign group Stonewall, said the importance of the change should not be underestimated.
"Our view is that civil partnerships are transformative for the lives of individual couples and their rights, but also for society more generally.
"Society now legally recognises gay relationships for the first time.
"It's a big day but 21 December, when the first partnerships take place, will be even bigger because that will see gay and lesbian people removing discrimination."
But a spokesman for the one of the UK's major Christian groups told the BBC they believed same-sex couples should not get the same rights as married couples.
"If you transport something unique, like marriage, into a different context, there's always a cost. And the cost here is in terms of reduction of marriage and the undermining of it," Don Horrocks of the Evangelical Alliance said.
Retailers are already beginning to cash in on the new partnerships.
A range of "Mr and Mr" and "Mrs and Mrs" cards will hit Asda supermarkets this week.
And sets of "Darling, Dearest, Queerest" embroidered towel and soaps went on sale at Superdrug stores on Friday.
Meanwhile, three short advertisements were published in the Births, Marriages and Deaths columns under the heading Civil partnerships in Monday's Times newspaper.
The heading is a new addition to the paper, which has carried family announcements for nearly 221 years.
Campaigners have focused on councils which have been equivocal about the new law.
Bromley in south-east London had initially planned not to offer public ceremonies. Lisburn in Northern Ireland also overturned a proposed ban.
Ms Munn said any councils dragging their feet needed to comply with the both the spirit and letter of the law.
"The legislation requires that every authority must offer a civil partnership. The basic level of that is a simple signing of a register - some couples may just want that alone.
"But if any councils are saying they won't allow [public] ceremonies, for couples who want that kind of celebration, then it's time they came into the 21st century."