Single parents of children over seven years old will only receive benefits if they are seeking work, the government announced today.
The changes would come into force in 2010, by which parents will enjoy "wraparound" child care, according to work and pensions secretary Peter Hain.
Under current policy, single parents do not have to look for work until their child is 16.
Mr Hain said the reform would "re-ignite the job crusade".
Lone parents will be "expected to make an eventual move into the labour market in return for new and more personalised support," he said.
He claimed the changes would work towards eliminating child poverty and, "in reaching out to the hardest to help, aims to offer true social mobility".
But the Tories said Labour had already been given enough chances to improve the benefit system, and the new proposals left them with a "distinct sense of déjà vu".
The Liberal Democrats did not criticise the content of the green paper, but pushed the government to ensure its promises on childcare provision were met before it forced single parents into work.
"The government has guaranteed itself headlines by cutting the entitlement of lone parents to income support, but if this policy is ever to work, access to affordable childcare must be radically improved," said Lib Dem work and pensions spokesman Danny Alexander.
"We need long term solutions to both employment support and the provision of childcare if we are to truly tackle poverty and unemployment."
The proposals could represent the first confrontation between Gordon Brown and his backbenchers.
He has enjoyed a welcome honeymoon period as party leader so far, with MPs basking in radically improved poll results and the effects of Mr Brown's widely-praised response to the terrorist attempts on London and Glasgow.
But Labour backbenches are commonly very hostile to welfare reform, and Mr Brown's free-market beliefs are rarely any closer to traditional Labour politics than those of Tony Blair.