A formal consultation on extending civil marriage to same sex couples will begin in 2012.
The public consultation "to consider how to make civil marriage available" will begin in March, and has been cautiously welcomed by gay rights activists.
The website Pink News quoted a Downing Street source who claimed David Cameron had "personally intervened" to bring the change forward.
Currently civil marriages are only to open to opposite sex couples, while civil partnerships are the preserve of same sex couples.
The consultation will look to extend marriage to same sex couples, although it will not do the same for opposite sex couples who want civil partnerships.
Minister for Equalities Lynne Featherstone said: "I am delighted to confirm that early next year, this government will begin a formal consultation on equal civil marriage for same-sex couples.
"This would allow us to make any legislative changes before the end of this Parliament.
"We will be working closely with all those who have an interest in the area to understand their views ahead of the formal consultation."
The gay and lesbian rights group Stonewall welcomed the announcement "as warmly as we have welcomed the previous two announcements that consultation was shortly to begin".
"We do regret that the Government appears to have once again delayed the timetable for consulting on this issue. However, we look forward to the Government honouring its pledge that this legislation will be passed by 2015."
Stonewall's draft response to the Government's consultation can be found here.
Other activists, including Peter Tatchell, have criticised the government's unwillingness to repeal the current ban on religious institutions performing gay marriages.
LibDems welcomed the move and indicated it could be a boost for the party ahead of next week's annual conference.
"It is good news that we're taking a big step forward in treating all Brits equally," said Mark Pack, a prominent blogger who worked for the party between 2000 and 2009. "It is a tribute to Lynne Featherstone's persistence."
The consultation will only cover civil marriage for same sex couples, and not religious marriage.