David Cameron has announced plans for a tax break for some married couples worth up to £200 a year.

Speaking before the Conservative party conference in Manchester, the prime minister said four million couples, including 15,000 in civil partnerships, would benefit from a £1,000 transferable tax allowance from 2015.

Couples will be able to apply online and will receive the benefit for the 2015-16 tax year by the summer of 2016.

The allowance will be available for straight and gay couples, as well as civil partners. It will apply if couples are both basic rate tax payers, with one spouse earning less than the personal allowance or not earning at all. At current rates, the couple would have to earn less than £41,451 a year to be eligible.

Cameron explained how the scheme will work in an article in the Daily Mail, saying: '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.'

Cameron originally promised tax breaks for married couples when he ran for the Conservative leadership in 2005, and it was also part of the Conservative election manifesto in 2010.

In his recent comments on the move, Cameron said the policy would reward 'commitment'. However the tax break, which will cost an estimated £600m, has been criticised by opposition MPs as a 'backward step'.

Rachel Reeves, shadow Treasury chief secretary, 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.'