Allowing gay people to marry will "make our society stronger", Prime Minister David Cameron said.
Mr Cameron's comments came in a last-minute televised statement recorded in 10 Downing Street less than two hours before MPs vote on a Bill to legalise same-sex marriage.
With dozens of Conservative MPs expected to oppose the measure in a free vote, Mr Cameron had to face some criticism for keeping a low profile on the issue.
But he insisted he had always been "vocal" in his support for the move.
"Today is an important day. I am a strong believer in marriage. It helps people commit to each other and I think it is right that gay people should be able to get married too," he said.
"This is, yes, about equality. But it is also about making our society stronger.
"I know there are strong views on both sides of the argument - I accept that. But I think this is an important step forward for our country."
Mr Cameron was speaking as a stormy debate on the second reading of the Marriage (Same Sex Couples) Bill drew to its conclusion in the House of Commons.
Introducing the Bill, Culture Secretary Maria Miller insisted that it would make Britain "a fairer place to live".
But she faced opposition from numerous Tory MPs, including the chairman of the backbench 1922 Committee Graham Brady, who said he had "serious misgivings" over ministers' assurances that religious freedom will not be compromised.