Cuts to state-funded help with childcare will cost 44,650 families in the West Midlands £416 each per year and force many working mothers to quit their jobs, a thinktank has warned.
Research organisation the Resolution Foundation produced figures showing the level of support that people will lose annually in each part of the country - up to £1,300 in parts of London.
Chancellor George Osborne announced in the spending review that the percentage of childcare costs recoverable through the working tax credit system would be reduced from 80% to 70% in April.
His move has been criticised by anti-poverty charities but the Foundation - which works to help low-to-middle earners - said many families remained unaware they would need to pay more. About 450,000 claim the assistance at present.
Resolution Foundation chief executive Gavin Kelly said: "For working mums on low-to-middle incomes losses like these will be hard to bear.
Many parents find support with childcare costs absolutely essential to staying in work, and the big worry is some will now find that work doesn't pay.
"With other work-related costs, such as train fares, now also rising, these cuts couldn't come at a worse time for families. Many families are still unaware of the changes, despite the fact that they come into effect in a few months' time.
"This was already set to be a tough year for families. With wages rising more slowly than inflation, households are going to get poorer, even before cuts are factored in."
Shadow work and pensions secretary Douglas Alexander said: "These new figures demonstrate just how hard families will be hit by George Osborne's cuts to childcare.
"Families are facing tough times, and these cuts to childcare will make it harder, not easier, for parents to get the work to help make ends meet."