The government grant to childcare providers for the existing 15 hours a week of free childcare is already "grossly underfunded", the Pre-School Learning Alliance has said.
Research for the charity suggests the government's plan to double this childcare would cost around £195bn a year, but funding at current rates amounts to £1.7bn - a potential shortfall of £250m.
Chief executive Neil Leitch said: "Simply raising funding rates by an arbitrary amount won't be enough - it is absolutely crucial that the Government ensures that the hourly rate of funding actually covers the cost of delivering funded places. Anything less risks destabilising a childcare system that is already struggling to stay afloat."
He warned an increasing number of providers may withdraw from the free childcare system if more money is not made available.
Speaking on the BBC Radio 4 Today programme, Mr Leitch added: "There are many nurseries that can't physically extend their number of hours. They may operate in a church hall or community centre. Nobody has considered whether in fact they will be able to offer the 30 hours."
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