Mandelson suggested a delay in extending flexible working from parents with children under six to parents with children under 16 because of the economic downturn.
But Kate Green, chief executive of the Child Poverty Action Group, said: "Flexible working rights are essential for thousands of lone parents due to be shifted onto Jobseekers Allowance (JSA) by the Government's welfare reforms.
"All along the welfare reforms were proposed on the basis that job would be there, private investment could be raised, child care would be there and employers would provide greater flexibility for parents. None of this is now in place.
"The Government must take urgent steps to postpone lone parents going onto JSA."
Sally Gimson, director of communications at the Family and Parenting Institute, said: "There is no denying that are we are facing uncertain times when it comes to the economy. But that doesn't mean we should drop all of the benefits we have fought so hard to get because times get tough.
"Being able to work flexibly can really help cut childcare costs. Many parents will lose out financially if these plans are delayed because they will have to pay for other arrangements."