Labour has accused the Government of watering down its scheme for 30 hours of free childcare after uncovering an apparent admission "hidden in the Budget" that the number of places could "rise more slowly" than anticipated.
In official Budget papers, the Government said: "It seems likely that the supply of places will will rise more slowly over the first two years than originally assumed."
It comes after a survey by the Family and Childcare Trust suggested last month that more than half of English councils are unsure if they will be able to provide enough childcare when plans to double the free hours available for pre-schoolers come into force in September.
The Government has estimated that 390,000 three and four-year-olds will qualify for the 30-hour offer, which is restricted to children from working families meeting certain earnings thresholds.
The Budget policy costings document says: "30 hours free childcare - this July 2015 measure is due to launch in September.
"As with TFC (tax-free childcare), we have made a small adjustment to the expected reduction in tax credits and associated welfare spending from the introduction of 30 hours of free childcare for working families, where it seems likely that the supply of places will rise more slowly over the first two years than originally assumed."
Shadow education secretary Angela Rayner said: "The cost of childcare has rocketed under the Tories and their promises to offer free hours have completely fallen flat.
"First we saw delay after delay to the tax-free childcare, and the extra free hours offer has been plagued with problems because the Tories have failed to fund it properly.
"Now there will be even fewer places available, so more children and their families will be losing out.
"The Tories just aren't delivering for ordinary working families."
Ellen Broome, deputy chief executive at the Family and Childcare Trust, said: "While we welcome Government's investment in childcare, we are not surprised that there is uncertainty about how many families will benefit from the extension of free childcare. This popular policy could leave many families disappointed if places are not available.
"Our childcare survey released earlier this month found that just a third of local authorities in England were confident that they would have enough childcare available for three and four-year-olds using the 30-hour offer.
"We need Government to put together a robust childcare strategy to better understand how to meet the childcare needs of parents and to make sure every child can get high-quality childcare that boosts their learning."