Jeremy Hunt has insisted the government is “on track” to deliver a planned expansion of free childcare in April.
However, the chancellor told the BBC he could not give “an absolute guarantee” that the places needed would all be available in time.
From April, the government has promised working parents of two-year-olds in England 15 hours of free care a week.
Labour said the commitment was “in tatters”, describing it as a “botched pledge without a plan”.
Under proposals announced in last year’s Budget, the government is rolling out the expansion of free childcare in stages.
Working parents of three and four-year-olds are already able to get 30 hours of free childcare.
From September, it will be extended to 15 hours for those aged nine months and above.
Then from September 2025, the government has pledged to offer 30 hours of free childcare for all under-fives.
However, nurseries and other early years providers have warned they will struggle to meet the expected demand for the scheme.
The sector argues that years of underfunding has led to a recruitment and retention crisis, meaning there are not enough staff to deliver the policy.
Nurseries have also complained of delays in confirming how much they will be paid to offer free hours.
Meanwhile, some parents have reported challenges finding local nurseries offering free hours which have places available.
Asked if he could give a guarantee to parents and childcare providers that all the places needed to deliver the pledge would be ready for April, Mr Hunt said the policy amounted to “the biggest expansion of childcare in a generation”.
“It may mean that we need to employ 40,000 more people in the sector and that’s why we’re bringing it in stages,” he told the BBC’s Sunday with Laura Kuenssberg programme.
“You’ll understand why I don’t want to give an absolute guarantee. But am I confident we are delivering this programme and it will be on track for this April? Yes, I am.”
Labour’s shadow education secretary Bridget Phillipson accused Mr Hunt of “rushing out” the commitment “with no plan to make it happen”.
“Now it’s working parents, who in the weeks and months ahead – when they go and try and access those places that have been promised – will discover that they just can’t be delivered in the way that he set out,” she told the programme.
She has written to the chancellor to demand the free childcare pledge will be delivered as scheduled and without parents facing extra charges.
Asked what provision Labour would guarantee if it wins power, Ms Phillipson said: “I’m not going to make the same mistakes the Conservatives have made – rushing something out without a plan to make it happen.”
She added her party had commissioned a review of early years by the former chief inspector of Ofsted, Sir David Bell, which she said would look at how to drive up standards and ensure childcare places were available.
Purnima Tanuku, chief executive of the National Day Nurseries Association, said the chancellor needed to address “the huge challenges facing early years providers”, including by offering sustainable funding rates and removing business rates.
“This is the only way that nurseries can plan for a sustainable future and be able to deliver the increased childcare offer being made to parents,” she said.
