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."
Couples will have to apply online for the new allowance, which is not available to those that include a higher rate taxpayer.
The reforms are aimed at couples where one partner has not used all of their personal allowance or does not work at all and are hoped to particularly benefit stay-at-home parents.
Mr Cameron tweeted: "The £1000 marriage tax allowance will apply to straight and gay couples, as well as civil partners. Love is love, commitment is commitment."
Tory high command will hope the move will appease backbenchers, who have repeatedly called for the policy to be enacted, as well as reach out to voters as the party heads to Manchester for its annual autumn gathering .
But, just as the Labour party suffered embarrassment following the publication of damaging memoirs by former spin doctor Damian McBride during its conference, Mr Cameron will have to contend with allegations over the way his government is run in a new book, In It Together by Matthew d'Ancona, serialised in The Daily Telegraph and The Sunday Telegraph.
It claims the PM has held talks with Deputy Prime Minister Nick Clegg the possibility of renewing their coalition in 2015 if the Conservatives fail to secure victory at the ballot box.
Discussions have taken place "from time to time" in the last 12 months, according to the book, which is said to be based on accounts from senior government figures.
Mr D'Ancona writes: "From time to time, he (Mr Cameron) would raise the question of a second coalition with Clegg. 'If we did it again,' he mused to the Deputy Prime Minister, 'I'd have to seek collective permission.'"
It also claims former strategy director Steve Hilton quit Downing Street after deciding Mr Cameron was a "reactive not transformative Prime Minister" while MI6 developed an escape plan for Muammar Gaddafi to flee for exile in Equatorial Guinea during the Libyan conflict.
A No 10 spokesman said: "There are not, nor have there been any plans or talks to form a coalition post 2015. The Prime Minister is entirely focused on running the country."
Shadow chief secretary to the treasury Rachel Reeves said: "David Cameron's so-called marriage tax break won't even help two-thirds of married couples, let alone millions of people who are separated, widowed or divorced. He's so out of touch he thinks people will get married for £3.85 a week.
"And even for the minority who might benefit, it will be far outweighed by what David Cameron's government has already taken away in higher VAT and cuts to child benefit and tax credits. In most cases, the extra payment will be paid to men, even though it is women who have disproportionately lost out so far.
"David Cameron also needs to explain how this policy will be paid for. Will he be raising taxes or making cuts elsewhere to fund this?
"At a time when millions of people are facing a cost of living crisis we should be helping all families and not just some. That's what Labour set out this week with plans to freeze energy bills and expand free childcare for working parents."
Liberal Democrat Treasury spokesman Stephen Williams said: "This is a tax cut for some, paid for by everyone else.
"Liberal Democrats are committed to helping everyone with the cost of living, single or married, and to deliver a tax system which is fair and progressive to everyone. That is why we have given a £700 tax cut to over 20 million working people across the UK.
"You don't build a fairer society by using the tax system to favour one type of family over another. We should support all families, not just the minority of married ones where one person stays at home. Extending marriage to as many people as possible is what's right and that's why we successfully campaigned for same-sex marriage, which I wholeheartedly support.
"All relationships are surely founded on love, not the tax system. It is also not clear to me why a single person should pay more tax on their income than someone who is married.
"The Liberal Democrats' priority is clear: in tough times we must support all families, not just a few. That is why we have prioritised funding free school meals for all infants, ensuring that children have the best possible start in life."
Mr Cameron arrived at the conference hotel at 6.30pm, where he was greeted by two would-be Tory MPs - Mary Robinson, the 2015 general election candidate for Cheadle and Christopher Green who will contest the Bolton West seat.
He did not stop to answer reporters' questions about his marriage tax plans.