Conservative plans to introduce a marriage tax break worth up to £150 a year for couples on low and middle incomes are facing criticism from Labour and the Liberal Democrats.
Under the proposals, the main breadwinner would be allowed to transfer £750 from their personal allowance to their spouse's.
This would help around four million married couples - just under a third of the total.
It would not apply to those earning more than £44,000, however. The tax break would be funded by a £1 billion levy on banks.
"Of course, some people don't want to get married and that's absolutely fine," Conservative leader David Cameron told the Daily Mail newspaper.
"We aren't saying people will get married because of money. But we think that sending a signal that families are at the heart of a strong society is a good thing to do.
Mr Cameron pointed out that a third of unmarried couples have separated by the time a child reaches its fifth birthday, compared to one in 11 for married couples.
He added: "I would like to do more. But we are in very straitened circumstances, so I see this as a first step, and it's about the message more than the money."
At a press conference at Labour party headquarters this morning, work and pensions secretary Yvette Cooper called the marriage tax break "unfair, irresponsible and out of date".
"We know families come in all shapes and sizes and it is wrong to tell children that their families are second-class just because their parents are not married," she said.
Ms Cooper claimed only 35% of those who would benefit actually have children - and pointed out the Tories plan to cut child tax credit.
"Many married couples with kids will actually be worse off," she explained. "If you take a married couple both working on average earnings with two kids they will actually be £540 a year worse off."
Labour proposes introducing 'toddler tax credits' which would give all low and middle income families with one- and two-year-olds giving them £4 extra a week.
Liberal Democrat leader Nick Clegg attacked Mr Cameron's proposals by calling them "patronising drivel that belong in the Edwardian age".
Launching his party's policy paper on families, he argued that the priorities should be flexibility for working families, high quality childcare and care for older people - as well as fairer taxation for all families and more affordable housing.
Mr Clegg said: "David Cameron clearly has no idea about modern life.
"Every family is different, and instead of creating rigid rules or special policies that help some families but not others, we need a new approach from government: one that is flexible and doesn't dictate to families how they should live."