Tax breaks for married couples worth up to £200 a year will be introduced from 2015, David Cameron has announced.
Ahead of the start of the Conservative Party conference, the Prime Minister pledged to introduce a £1,000 transferable tax allowance that will benefit four million couples .
The move, which comes after a trade-off that allowed the Liberal Democrats to announce free school meals for all children under eight earlier this month, makes good on a Conservative manifesto commitment as well as promises Mr Cameron made when he was running for leadership of the party in 2005.
Writing in the Daily Mail, Mr Cameron said: "I believe in marriage. Alongside the birth of my children, my wedding was the happiest day of my life.
"Since then, Samantha and I have been a team. Nothing I've done since - becoming a Member of Parliament, leader of my party or Prime Minister - would have been possible without her.
"There is something special about marriage: it's a declaration of commitment, responsibility and stability that helps to bind families.
"The values of marriage are give and take, support and sacrifice - values that we need more of in this country."
The scheme will start in April 2015 and will apply to the 15,000 couples in civil partnerships. They will receive the benefit from the scheme at the end of the tax year in 2016.
Mr Cameron added: " When I ran for the leadership of my party back in 2005, I said that I wanted to do more for marriage in the tax system: a personal pledge that I made right at the start of my campaign - and I then backed that up with a pledge in our manifesto at the last election.
"So this week at the Conservative Party's conference in Manchester, I'm going to deliver on the promise I made.
"From April 2015, if neither of you are higher rate taxpayers, you will be able to transfer £1,000 of your tax free allowance to your spouse.
"In effect, if you pay the basic rate of tax and your partner doesn't use all of their personal allowance, you'll be able to have some of it. Most couples who benefit will be £200 a year better off as a result.
"And of course this will be true if you're gay or straight - and in a civil partnership or a marriage. This summer I was proud to make Equal Marriage the law. Love is love, commitment is commitment."