Thousands of couples are expected to take advantage of rules which will allow them to convert their civil partnership into a same-sex marriage.
To celebrate the new arrangements, which come into force today (December 10th), Transport for London has rolled out a rainbow-coloured crossing in Trafalgar Square.
The date was also hailed by gay rights charity Stonewall, who said it had been eagerly awaited by thousands of couples nationwide.
The Government has already announced that for the next year those couples who entered into civil partnerships prior to gay marriage becoming law, will be entitled to a £45 fee reduction.
This will make it free for those who decide to hold a conversion ceremony at a register office.
Welcoming the new rules, Deputy Prime Minister Nick Clegg said: “I am proud that couples can finally convert their civil partnership to marriage.
“Today is another celebration for LGBT rights in this country, but as ever it’s a reminder that there are people across the world living under much less fortunate conditions, facing discrimination and persecution on a daily basis.
“So as we raise a glass to those making their vows, we send a message of solidarity to LGBT people everywhere.”
In total, more than 60,000 civil partnerships have been formed over the past decade with a substantial number of these expected to be converted to marriages in the coming months.
Parties are only required to make a short declaration that they know of “no legal reason why we may not convert our civil partnership into a marriage.”
A couple will then receive backdated marriage certificates and the marriage will be judged to have legally commenced on the date of the original civil partnership.