David Cameron said he would oppose the proposal
Tory leader David Cameron has described as "shameful" government proposals to make single mothers with very young children have to prepare for work.
Mr Cameron said it was wrong parents should be put in this position before their children go to school, adding he would pressure ministers to drop it.
The Tories back Labour's wider plan to get people off benefits and into work.
But Labour said Mr Cameron's latest comments showed he was "happy" for people on benefits "to do nothing".
The Tories had pledged to support the government's welfare reform bill which would see sanctions, including cuts in benefits, imposed on people who repeatedly turn down jobs or refuse to seek work.
Ministers want virtually everyone, except the seriously ill and disabled and those with children aged under one, to be either seeking work or preparing themselves for an eventual return to the workplace.
Parents with children aged between one and seven will be expected to start preparing for work - for example by tackling health or debt issues, attending work-focused interviews and agreeing an "action plan" to get them back into work.
But Mr Cameron described this idea as "a rotten apple in the barrel".
He said he did not oppose practical proposals to help parents such as debt or drug counselling but the "sensible" time for them to think about work was when their children were at school.
"We need to help families not make life harder for them," he said at his monthly news conference.
He said he suspected the proposal, which is likely be opposed by some Labour MPs, was being driven by a "macho positioning exercise" from ministers which he said was "pretty sick".
"It potentially hits single mothers at a very vulnerable time of their lives," he said, adding he wanted the idea "taken out" of the bill.
Ministers said the Tories were "wrong" on what steps were needed to reform the welfare system.
"David Cameron bangs on about Britain being broken but he would do nothing to help people off benefits," Work and Pensions Secretary James Purnell said.
"We want everyone on benefits to do something in return for their money, including asking lone parents to take active steps to prepare for a return to work. He is happy for people to do nothing.
"He would make the same mistakes the Tories did in the past - doing nothing to require them to take up that help."
The Lib Dems also criticised Mr Cameron for what they said was his "belated" understanding of the difficulties faced by lone parents.
"It is clear public opinion is against persecuting single parents yet it has taken the Conservatives a week to fall in step with popular sentiment," its work and pensions spokesman Jenny Willott said.
Ministers have pledged extra investment in employment advice and childcare in return for .
But charities have expressed concerns that single mothers with young children could be stigmatised by the proposals.
While supporting the government's wider aim of reducing welfare dependency and getting more people into work, the Conservatives have also criticised ministers for past inaction.