Tory plans to give married couples a tax break have been described as "social engineering" by a Cabinet minister as the Government prepares to unveil a series of family-friendly policies.

Schools Secretary Ed Balls laid into David Cameron's pledge to recognise marriage financially, warning it would create a tier of "second class" families which risked stigmatising children.

Extra cash for the Relate service and a duty on public services to prioritise families were reported to be among measures to be promised by ministers in a Green Paper next week.

It will also include efforts to give more support to fathers, including a 10-page "New Dad's Guide" and new guidance to midwives on how to better engage with fathers.

As Labour drew battle lines with the Opposition on the issue, Mr Balls said it was vital for the state to recognise that good families came "in all shapes and sizes".

"The idea of trying to socially engineer family life through a tax policy which is designed to say that some types of families are first class, and other types of families are second class and should be financially disadvantaged, is hugely expensive and unfair," he said.

"The idea that you say to children who only have one parent because of bereavement or domestic violence, or to kids who have two parents, both of whom are divorced and who have a new family but don't want to or can't remarry that somehow you're not as good as another type of family is unfair. It could stigmatise children and I don't think that's right. I don't think children should be told they are second-class kids because of things which have happened through no fault of their own or unavoidable reasons."

Mr Balls, the husband of Work and Pensions Secretary Yvette Cooper, said he believed marriage remained "the best way to bring up children" but it was wrong to penalise other family units.

Mr Cameron was forced to renew his promise to deliver the tax break "within a Parliament" last week after appearing to downgrade the commitment to a "hope". He later issued a statement saying that the party was, in fact, pledged to deliver on the policy in its first term and said he had "messed up" by "misdescribing" the position.

Shadow chief secretary to the Treasury Philip Hammond defended the policy. He said: "Creating a framework of support for families is something that a strong and confident society should do, and that includes recognition of marriage in the tax system, respecting the importance of extended families, and acknowledging the value that stable families bring to our communities. There is plenty of evidence that being brought up in a stable family unit is one of the most important indicators of a child's future prospects. So encouraging stable families is a key tool in the fight against intergenerational poverty."