The Civil Partnership Act 2004 comes into force today, enabling gay couples to obtain legal recognition of their relationships.
For the first time, same sex couples will the same rights as married couples on employment, income, tax and pension benefits, and will be able to apply for rights over a partner's children.
However, unlike in Belgium, the Netherlands, Spain and Canada, gay couples in Britain will not hold the legal title of marriage, and only sign exclusively civil documents, rather than exchange spoken words in a civil or religious ceremony.
Although the law comes into effect today, the first legally-permitted gay ceremonies will not be carried out until December 21st, because of the requirement for couples to give notice to registry offices.
Thousands of ceremonies are already thought to have been booked for that date, including that of Elton John and his long-term partner, David Furnish.
In an interview with gay magazine Attitude, the singer said: "We have no plans to do anything spectacular. It's just going to be done in a dignified way... The ceremony will be very private, a small family affair."
Although the law changes have been welcomed by most, a number of local authorities had initially refused to allow the partnership ceremonies on council premises.
One of these was the Conservative-controlled Bromley council, in south London, although it has backtracked on the decision following pressure from London major Ken Livingstone.
In Northern Ireland, where homosexuality was only made legal 15 years after the rest of the UK in 1982, the Democratic Unionist party (DUP), led by Ian Paisley, ran a Save Ulster from Sodomy campaign.