Former Prime Minister Sir John Major said: "The Prime Minister's instinct to support equal marriage is a courageous and genuine attempt to offer security and comfort to people who - at present - may be together, yet feel apart.
"I fully understand that there are many who will find this difficult to accept, as will the Churches.
"But the Prime Minister has made it clear that the Churches will be free to make their own decisions upon whether to conduct such marriages - and that is entirely the right approach.
"We live in the 21st century and must move on: every couple should have the opportunity and the right to formalise their relationship."
Same-sex marriage will remain illegal in the Church of England, as the Government unveiled its plans to extend marriage to gay couples.
David Cameron is facing a backlash from Tory MPs and religious groups over allowing gay weddings in churches.