Liberal Democrat party members will vote on full marriage equality at their annual conference next month.
The motion will call for support for gay marriage to become official party policy and is likely to pass.
It says that civil partnerships should be converted to marriage for those who wish to be married, while marriage and civil partnerships should be available to everyone.
It adds that transgender people should be allowed to remain married after changing sex.
While grassroots members are expected to support the move, it may be contentious with more senior members of the party.
A Liberal Democrat source told the Independent on Sunday: “There will undoubtedly be some people that will speak against it, especially from the various religious groups. But this is something that the party as a whole has been calling for. It will be a key issue for us in defining ourselves against the Tories.”
Gay Liberal Democrat MP Stephen Gilbert, who is supporting the motion, said: “The Liberal Democrats have long fought for LGBT equality and were the first party with a gay rights policy. With the support of Stonewall we were also the first party to introduce a Civil Partnership Bill. And now I hope we can continue this record and become the first party to officially support full marriage equality.
“The time has come to take the final step and end the slow, difficult and long-overdue campaign for gay marriage. I believe that with the backing of our party the Liberal Democrats in government will be able to grant same-sex couples the equality and freedoms that are rightfully theirs.”
The motion will be voted on immediately before the party’s new deputy leader Simon Hughes, who is bisexual, makes his keynote speech.
Supporters of the motion cite party leader Nick Clegg’s comments to PinkNews.co.uk in February, when he said he agreed with gay marriage.
At the time, Mr Clegg said: “I support gay marriage. Love is the same, straight or gay, so the civil institution should be the same too. All couples should be able to make that commitment to one another.”
The Liberal Democrat conference, to be held in Liverpool, will be the party’s first conference in government.
The coalition government supports allowing religious premises to hold civil partnerships if they wish and ministers have begun consultations with gay rights groups on the issue.