The Liberal Democrat Autumn Conference yesterday passed a motion calling for the opening up of marriage and civil partnerships to both same-sex and mixed-sex couples.
The motion also called for allowing same-sex religious marriage ceremonies and giving people the right to stay married or civilly partnered when they legally change their gender.
As an official party policy, Lib Dem ministers will now be expected to lobby for the legal change. However, the issues may divide the Coalition Government as although David Cameron said before the General Election that he was happy to discuss the issue, he also said that he has no immediate plans to change the definition of marriage.
Lib Dem equalities minister Lynne Featherstone said she was proud of her party for backing the move, but said it was "a very sensitive issue" and she would take time to listen to all parts of the community before pressing for change.
Speaking to the The Daily Telegraph, Ms Featherstone said: "I have always been in favour of complete equality. However as a Government minister it is not a matter of simply steaming ahead in one particular direction.
"There is no decision yet apart from understanding from all perspectives what next steps will look like and what the ramifications would mean. The marriage laws have not been touched for 200 years and there are some real difficult issues within the law."
Although the Civil Partnership Act, passed in 2004, gives many of the rights and privileges of marriage to same-sex couples, the law stops short of full marriage.
Commenting, Lib Dem MP Stephen Gilbert said: "Current legislation degrades same-sex couples to a second-tier partnership and leads to unnecessary pain and trouble for anyone wishing to change their legally recognised gender, forcing them to divorce or dissolve their civil partnership and enter into a different commitment.
"It is time that Britain ends the current unfair legal situation and regains its position as a country leading the fight for full LGBT equality."