Speaking at the Conservative Party Conference on Wednesday, Miller responded to the Gordon Brown's announcement last week promising to provide 10 hours of free childcare a week for 250,000 two-year olds from families "on modest or middle incomes" within five years.
Miller said: " It is hard sustaining family life... The prime minister is axing [tax efficiencies on] childcare that 300,000 working parents need.
"These are basic rate taxpayers, working in areas such as the health sector and doing vital jobs. They rely on this support.
"Hard-working families will be left to pick up the pieces and this has got to stop."
A Conservative Government will support families, building systems around them for childcare."
Marc Woolfson, account director at Westminster Advisers, told HR magazine 340,000 parents and 35,000 employers currently use tax-efficient childcare vouchers and a report earlier this year from the Department of Children Schools and Families found such vouchers were ‘very popular' with parents and employers.