The deal is part of a proposed $1,600 transferable tax allowance from 2015 and would apply to couples making $66,200 per year or less -- or those who fall into Britain's 20 percent basic tax rate.
Cameron's Conservative government proposed the tax break as a means to support the institution of marriage.
It would apply to both straight and gay couples and some people in long-term committed relationships who aren't married, the BBC said Saturday.
"I believe in marriage. Alongside the birth of my children, my wedding was the happiest day of my life," Cameron said.
Liberal Democrats in his government coalition opposed the measure but agreed not to vote against it in exchange for Conservatives agreeing to a free school meal program for children age 8 and younger.
Opposition Labor party members blasted Cameron's pledge.
"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," shadow Treasury Secretary Rachael Reeves said. "He's so out of touch he thinks people will get married for [$6.17] a week."