The Government will hold a consultation on redefining marriage in England and Wales to allow homosexuals to wed, following similar moves in Scotland.
The announcement was made at the weekend by equalities minister Lynne Featherstone.
Opponents say there will be far-reaching consequences if a key social institution is so radically rewritten.
The consultation will begin in the spring. The Government says it will ask how, not if, same-sex marriage should be legalised.
Downing Street sources claim David Cameron’s personal intervention has pushed the issue forward.
But Lynne Featherstone, a Lib Dem, insists the announcement is a triumph for her party within the coalition Government.
The Prime Minister has been put under pressure to redefine marriage by homosexual campaigners, like Peter Tatchell.
Mr Tatchell has a group of court cases heading for the European Court of Human Rights in an attempt to push same-sex marriage onto Britain.
One of his legal actions includes a challenge by anti-marriage heterosexuals who demand the right to have a civil partnership.
The Government has confidently asserted that it will have the legislation on the statute book by the year 2015.
But many backbench MPs and Peers are normally allowed a free vote on controversial matters such as this.
There is opposition to redefining marriage from within the Tory party. Grandee Lord Tebbit said: “there were other priorities at a time like this”.
He added that “there can be no such thing as gay marriage. Marriage is between a man and a woman to the exclusion of all others”.
And Tory MEP Roger Helmer said: “it is not the business of government to legislate to change the meaning of a common and well-established word, and least of all a word that describes such a key institution in society”.
He added: “If marriage becomes broader, it becomes shallower, and the vital importance of marriage in our society will be further eroded.”
In February senior Tory backbencher Edward Leigh MP said: “Same-sex couples already have all the rights of marriage in the form of civil partnership.
“Why must they also have the language of marriage? No doubt because it is an important symbol to them. But it is also an important symbol to many other people.
“Must the religious and cultural heritage of the whole nation be overturned to suit the demands of a minority even of the gay community itself?”
He added: “A recognisably Conservative Government would not do this.”
The Christian Institute’s Mike Judge said: “The census shows that only one household in 500 is headed by a same-sex couple. Is marriage to be redefined across the whole of society to please a small minority of activists?
“If marriage is redefined for homosexual marriage, that new definition will be the one that the state promotes as the standard. It will have huge implications for what is taught in schools and for wider society.”
But equalities minister Lynne Featherstone said Britain is “a world leader for gay rights but there is still more we must do.” She vowed to push same-sex marriage through into law before the next general election.