The government has said it is committed to changing the law in England and Wales to allow gay marriage by 2015.
Ministers are to launch a consultation next spring on how to open up civil marriage to same-sex couples ahead of the next general election.
Equalities Minister Lynne Featherstone told the Lib Dem autumn conference that current laws were "simply not fair".
The leadership of both coalition parties back the move but it is likely to anger some Conservative activists.
And gay rights campaigners have urged the government to act immediately, saying existing laws are discriminatory and a consultation is unnecessary.
At the moment, only men and women are permitted to get married while civil partnerships, which became law in 2005, are limited to same-sex couples.
Civil partnerships give same-sex couples the right to the same legal treatment across a range of matters as married couples but the law does not allow such unions to be referred to as marriages.
Ms Featherstone told Lib Dem activists that a public consultation will begin in March 2012 with a view to changing the law ahead of the next general election scheduled for May 2015.
"Britain must not be complacent," she said. "We are a world leader for gay rights but there is still more we must do."
While civil partnerships were a "welcome first step", she said the party was committed to confronting "prejudice and discrimination in all its forms".
Heralding the proposed change as a Lib Dem policy, she added: "To deny one group of people the same opportunities available to another is not simply discriminatory. It is simply not fair."
Ministers have said the government is determined to listen to "all those who have an interest in the area to understand their views".
However, the consultation will not consider whether to allow same-sex couples to have religious marriages or to open up civil partnerships to men and women.
The Lib Dems have long campaigned for reform of the marriage laws, arguing that they are outdated and discriminate against same-sex couples.
Conservative leader David Cameron backed the move while in opposition as part of his modernising drive and the pledge to permit equal marriage was included in the party's 2010 election manifesto.
However, some Conservative MPs and activists are likely to be uncomfortable with the move.
And veteran gay rights campaigner Peter Tatchell said the government had delayed the consultation - which campaigners had expected to start last June.
"I am not convinced that there needs to be any consultation at all," he said. "The ban on same-sex marriage is homophobic discrimination and should be repealed."
And he said same-sex couples should be allowed to wed in churches and other religious buildings, arguing that some faith organisations had expressly asked to be able to conduct same-sex ceremonies.
He added: "It is an insult to people of faith for the equality minister to rule out any repeal of the ban on religious organisations conducting same-sex marriages."
For Labour, Yvette Cooper said the party would "keep the pressure on this government to enact measures like this".
But she accused the government of having "said one thing on equality and done another - including dragging their feet on civil partnerships on religious premises".
"So we will have to see whether this amounts to a real commitment or just an announcement for Lib Dem party conference that will not be implemented in this Parliament."
New rules set to come into force early next year will allow religious premises to hold civil partnership ceremonies. The move is voluntary and religious organisations will not be obliged to do so.
A group of British couples are challenging the existing ban on gay marriages and heterosexual civil partnerships and plan to take the case to the European Court of Justice.
New York became the largest US state to date to legalise gay marriage earlier this year. Same-sex marriage is legal in six states and Washington DC.