David Cameron says four million couples will benefit from the Government's new £1,000 marriage tax allowance.
Ahead of the start of the Conservative Party conference, the Prime Minister said the scheme - starting in April 2015 - will be worth up to £200 a year for married couples, including 15,000 in civil partnerships.
They will receive the benefit at the end of the tax year in 2016.
It will work by letting people transfer £1,000 of their personal tax allowance to their spouse or civil partner - an increase on the £750 allowance promised in the Tory manifesto, which would have seen couples gain £150.
The new allowance, which is not available to couples which include a higher rate taxpayer, is aimed at couples where one partner has not used all of their personal allowance or does not work at all.
Labour's shadow chief secretary to the treasury, Rachel Reeves, said that the marriage tax break would not even help two-thirds of married couples and said he was out of touch if he "thinks people will get married for £3.85 a week".
She said: "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."
The PM says 'nothing would be possible' without his wife Samantha The announcement comes after a trade-off that allowed the Liberal Democrats to announce free school meals for all children under eight earlier this month.
The proposal, which Downing Street said shows the Government values commitment by recognising marriage and civil partnerships in the tax system, makes good on promises Mr Cameron made when he was running for leadership of the party in 2005.
In an article in today's edition of The Daily Mail, he 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."
He said that the new measures would apply "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".
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